1. Spring Web MVC

1.1. Introduction

Spring Web MVC is the original web framework built on the Servlet API and included
in the Spring Framework from the very beginning. The formal name "Spring Web MVC"
comes from the name of its source module
spring-webmvc
but it is more commonly known as "Spring MVC".

1.2. DispatcherServlet

Spring MVC, like many other web frameworks, is designed around the front controller
pattern where a central Servlet, the DispatcherServlet, provides a shared algorithm
for request processing while actual work is performed by configurable, delegate components.
This model is flexible and supports diverse workflows.

The DispatcherServlet, as any Servlet, needs to be declared and mapped according
to the Servlet specification using Java configuration or in web.xml.
In turn the DispatcherServlet uses Spring configuration to discover
the delegate components it needs for request mapping, view resolution, exception
handling, and more.

Below is an example of the Java configuration that registers and initializes
the DispatcherServlet. This class is auto-detected by the Servlet container
(see Servlet Config):

Spring Boot follows a different initialization sequence. Rather than hooking into
the lifecycle of the Servlet container, Spring Boot uses Spring configuration to
bootstrap itself and the embedded Servlet container. Filter and Servlet declarations
are detected in Spring configuration and registered with the Servlet container.
For more details check the
Spring Boot docs.

1.2.1. Context Hierarchy

DispatcherServlet expects a WebApplicationContext, an extension of a plain
ApplicationContext, for its own configuration. WebApplicationContext has a link to the
ServletContext and Servlet it is associated with. It is also bound to the ServletContext
such that applications can use static methods on RequestContextUtils to look up the
WebApplicationContext if they need access to it.

For many applications having a single WebApplicationContext is simple and sufficient.
It is also possible to have a context hierarchy where one root WebApplicationContext
is shared across multiple DispatcherServlet (or other Servlet) instances, each with
its own child WebApplicationContext configuration.
See Additional Capabilities of the ApplicationContext
for more on the context hierarchy feature.

The root WebApplicationContext typically contains infrastructure beans such as data repositories and
business services that need to be shared across multiple Servlet instances. Those beans
are effectively inherited and could be overridden (i.e. re-declared) in the Servlet-specific,
child WebApplicationContext which typically contains beans local to the given Servlet:

Below is example configuration with a WebApplicationContext hierarchy:

1.2.2. Special Bean Types

The DispatcherServlet delegates to special beans to process requests and render the
appropriate responses. By "special beans" we mean Spring-managed, Object instances that
implement WebFlux framework contracts. Those usually come with built-in contracts but
you can customize their properties, extend or replace them.

The table below lists the special beans detected by the DispatcherHandler:

Map a request to a handler along with a list of
interceptors for pre- and post-processing.
The mapping is based on some criteria the details of which vary by HandlerMapping
implementation.

The two main HandlerMapping implementations are RequestMappingHandlerMapping which
supports @RequestMapping annotated methods and SimpleUrlHandlerMapping which
maintains explicit registrations of URI path patterns to handlers.

HandlerAdapter

Help the DispatcherServlet to invoke a handler mapped to a request regardless of
how the handler is actually invoked. For example, invoking an annotated controller
requires resolving annotations. The main purpose of a HandlerAdapter is
to shield the DispatcherServlet from such details.

1.2.3. Web MVC Config

Applications can declare the infrastructure beans listed in Special Bean Types
that are required to process requests. The DispatcherServlet checks the
WebApplicationContext for each special bean. If there are no matching bean types,
it falls back on the default types listed in
DispatcherServlet.properties.

In most cases the MVC Config is the best starting point. It declares the required
beans in either Java or XML, and provides a higher level configuration callback API to
customize it.

Spring Boot relies on the MVC Java config to configure Spring MVC and also
provides many extra convenient options.

1.2.4. Servlet Config

In a Servlet 3.0+ environment, you have the option of configuring the Servlet container
programmatically as an alternative or in combination with a web.xml file. Below is an
example of registering a DispatcherServlet:

WebApplicationInitializer is an interface provided by Spring MVC that ensures your
implementation is detected and automatically used to initialize any Servlet 3 container.
An abstract base class implementation of WebApplicationInitializer named
AbstractDispatcherServletInitializer makes it even easier to register the
DispatcherServlet by simply overriding methods to specify the servlet mapping and the
location of the DispatcherServlet configuration.

This is recommended for applications that use Java-based Spring configuration:

Each filter is added with a default name based on its concrete type and automatically
mapped to the DispatcherServlet.

The isAsyncSupported protected method of AbstractDispatcherServletInitializer
provides a single place to enable async support on the DispatcherServlet and all
filters mapped to it. By default this flag is set to true.

Finally, if you need to further customize the DispatcherServlet itself, you can
override the createDispatcherServlet method.

1.2.5. Processing

The WebApplicationContext is searched for and bound in the request as an attribute
that the controller and other elements in the process can use. It is bound by default
under the key DispatcherServlet.WEB_APPLICATION_CONTEXT_ATTRIBUTE.

The locale resolver is bound to the request to enable elements in the process to
resolve the locale to use when processing the request (rendering the view, preparing
data, and so on). If you do not need locale resolving, you do not need it.

The theme resolver is bound to the request to let elements such as views determine
which theme to use. If you do not use themes, you can ignore it.

If you specify a multipart file resolver, the request is inspected for multiparts; if
multiparts are found, the request is wrapped in a MultipartHttpServletRequest for
further processing by other elements in the process. See Multipart resolver for further
information about multipart handling.

An appropriate handler is searched for. If a handler is found, the execution chain
associated with the handler (preprocessors, postprocessors, and controllers) is
executed in order to prepare a model or rendering. Or alternatively for annotated
controllers, the response may be rendered (within the HandlerAdapter) instead of
returning a view.

If a model is returned, the view is rendered. If no model is returned, (may be due to
a preprocessor or postprocessor intercepting the request, perhaps for security
reasons), no view is rendered, because the request could already have been fulfilled.

The HandlerExceptionResolver beans declared in the WebApplicationContext are used to
resolve exceptions thrown during request processing. Those exception resolvers allow
customizing the logic to address exceptions. See Exceptions for more details.

The Spring DispatcherServlet also supports the return of the
last-modification-date, as specified by the Servlet API. The process of determining
the last modification date for a specific request is straightforward: the
DispatcherServlet looks up an appropriate handler mapping and tests whether the
handler that is found implements the LastModified interface. If so, the value of the
long getLastModified(request) method of the LastModified interface is returned to
the client.

You can customize individual DispatcherServlet instances by adding Servlet
initialization parameters ( init-param elements) to the Servlet declaration in the
web.xml file. See the following table for the list of supported parameters.

Table 1. DispatcherServlet initialization parameters

Parameter

Explanation

contextClass

Class that implements WebApplicationContext, which instantiates the context used by
this Servlet. By default, the XmlWebApplicationContext is used.

contextConfigLocation

String that is passed to the context instance (specified by contextClass) to
indicate where context(s) can be found. The string consists potentially of multiple
strings (using a comma as a delimiter) to support multiple contexts. In case of
multiple context locations with beans that are defined twice, the latest location
takes precedence.

namespace

Namespace of the WebApplicationContext. Defaults to [servlet-name]-servlet.

throwExceptionIfNoHandlerFound

Whether to throw a NoHandlerFoundException when no handler was found for a request.
The exception can then be caught with a HandlerExceptionResolver, e.g. via an
@ExceptionHandler controller method, and handled as any others.

By default this is set to "false", in which case the DispatcherServlet sets the
response status to 404 (NOT_FOUND) without raising an exception.

Note that if default servlet handling is
also configured, then unresolved requests are always forwarded to the default servlet
and a 404 would never be raised.

1.2.6. Interception

All HandlerMapping implementations supports handler interceptors that are useful when
you want to apply specific functionality to certain requests, for example, checking for
a principal. Interceptors must implement HandlerInterceptor from the
org.springframework.web.servlet package with three methods that should provide enough
flexibility to do all kinds of pre-processing and post-processing:

preHandle(..) — before the actual handler is executed

postHandle(..) — after the handler is executed

afterCompletion(..) — after the complete request has finished

The preHandle(..) method returns a boolean value. You can use this method to break or
continue the processing of the execution chain. When this method returns true, the
handler execution chain will continue; when it returns false, the DispatcherServlet
assumes the interceptor itself has taken care of requests (and, for example, rendered an
appropriate view) and does not continue executing the other interceptors and the actual
handler in the execution chain.

See Interceptors in the section on MVC configuration for examples of how to
configure interceptors. You can also register them directly via setters on individual
HandlerMapping implementations.

Note that postHandle is less useful with @ResponseBody and ResponseEntity methods for
which the response is written and committed within the HandlerAdapter and before
postHandle. That means its too late to make any changes to the response such as adding
an extra header. For such scenarios you can implement ResponseBodyAdvice and either
declare it as an Controller Advice bean or configure it directly on
RequestMappingHandlerAdapter.

1.2.7. Exceptions

If an exception occurs during request mapping or is thrown from a request handler such as
an @Controller, the DispatcherServlet delegates to a chain of HandlerExceptionResolver
beans to resolve the exception and provide alternative handling, which typically is an
error response.

The table below lists the available HandlerExceptionResolver implementations:

Table 2. HandlerExceptionResolver implementations

HandlerExceptionResolver

Description

SimpleMappingExceptionResolver

A mapping between exception class names and error view names. Useful for rendering
error pages in a browser application.

Resolves exceptions raised by Spring MVC and maps them to HTTP status codes.
Also see alternative ResponseEntityExceptionHandler and REST API exceptions.

ResponseStatusExceptionResolver

Resolves exceptions with the @ResponseStatus annotation and maps them to HTTP status
codes based on the value in the annotation.

ExceptionHandlerExceptionResolver

Resolves exceptions by invoking an @ExceptionHandler method in an @Controller or an
@ControllerAdvice class. See @ExceptionHandler methods.

Chain of resolvers

You can form an exception resolver chain simply by declaring multiple HandlerExceptionResolver
beans in your Spring configuration and setting their order properties as needed.
The higher the order property, the later the exception resolver is positioned.

The contract of HandlerExceptionResolver specifies that it can return:

ModelAndView that points to an error view.

Empty ModelAndView if the exception was handled within the resolver.

null if the exception remains unresolved, for subsequent resolvers to try; and if the
exception remains at the end, it is allowed to bubble up to the Servlet container.

The MVC Config automatically declares built-in resolvers for default Spring MVC
exceptions, for @ResponseStatus annotated exceptions, and for support of
@ExceptionHandler methods. You can customize that list or replace it.

Container error page

If an exception remains unresolved by any HandlerExceptionResolver and is therefore
left to propagate, or if the response status is set to an error status (i.e. 4xx, 5xx),
Servlet containers may render a default error page in HTML. To customize the default
error page of the container, you can declare an error page mapping in web.xml:

<error-page><location>/error</location></error-page>

Given the above, when an exception bubbles up, or the response has an error status, the
Servlet container makes an ERROR dispatch within the container to the configured URL
(e.g. "/error"). This is then processed by the DispatcherServlet, possibly mapping it
to an @Controller which could be implemented to return an error view name with a model
or to render a JSON response as shown below:

1.2.8. View Resolution

Spring MVC defines the ViewResolver and View interfaces that enable you to render
models in a browser without tying you to a specific view technology. ViewResolver
provides a mapping between view names and actual views. View addresses the preparation
of data before handing over to a specific view technology.

The table below provides more details on the ViewResolver hierarchy:

Table 3. ViewResolver implementations

ViewResolver

Description

AbstractCachingViewResolver

Sub-classes of AbstractCachingViewResolver cache view instances that they resolve.
Caching improves performance of certain view technologies. It’s possible to turn off the
cache by setting the cache property to false. Furthermore, if you must refresh a
certain view at runtime (for example when a FreeMarker template is modified), you can use
the removeFromCache(String viewName, Locale loc) method.

XmlViewResolver

Implementation of ViewResolver that accepts a configuration file written in XML with
the same DTD as Spring’s XML bean factories. The default configuration file is
/WEB-INF/views.xml.

ResourceBundleViewResolver

Implementation of ViewResolver that uses bean definitions in a ResourceBundle,
specified by the bundle base name, and for each view it is supposed to resolve, it uses
the value of the property [viewname].(class) as the view class and the value of the
property [viewname].url as the view url. Examples can be found in the chapter on
View Technologies.

UrlBasedViewResolver

Simple implementation of the ViewResolver interface that effects the direct
resolution of logical view names to URLs, without an explicit mapping definition. This
is appropriate if your logical names match the names of your view resources in a
straightforward manner, without the need for arbitrary mappings.

InternalResourceViewResolver

Convenient subclass of UrlBasedViewResolver that supports InternalResourceView (in
effect, Servlets and JSPs) and subclasses such as JstlView and TilesView. You can
specify the view class for all views generated by this resolver by using
setViewClass(..). See the UrlBasedViewResolver javadocs for details.

FreeMarkerViewResolver

Convenient subclass of UrlBasedViewResolver that supports FreeMarkerView and
custom subclasses of them.

ContentNegotiatingViewResolver

Implementation of the ViewResolver interface that resolves a view based on the
request file name or Accept header. See Content negotiation.

Handling

You chain view resolvers by declaring more than one resolver beans and, if necessary, by
setting the order property to specify ordering. Remember, the higher the order property,
the later the view resolver is positioned in the chain.

The contract of a ViewResolver specifies that it can return null to indicate the
view could not be found. However in the case of JSPs, and InternalResourceViewResolver,
the only way to figure out if a JSP exists is to perform a dispatch through
RequestDispatcher. Therefore an InternalResourceViewResolver must always be configured
to be last in the overall order of view resolvers.

To configure view resolution is as simple as adding ViewResolver beans to your Spring
configuration. The MVC Config provides provides a dedicated configuration API for
View Resolvers and also for adding logic-less
View Controllers which are useful for HTML template
rendering without controller logic.

Redirecting

The special redirect: prefix in a view name allows you to perform a redirect. The
UrlBasedViewResolver (and sub-classes) recognize this as an instruction that a
redirect is needed. The rest of the view name is the redirect URL.

The net effect is the same as if the controller had returned a RedirectView, but now
the controller itself can simply operate in terms of logical view names. A logical view
name such as redirect:/myapp/some/resource will redirect relative to the current
Servlet context, while a name such as redirect:http://myhost.com/some/arbitrary/path
will redirect to an absolute URL.

Note that if a controller method is annotated with the @ResponseStatus, the annotation
value takes precedence over the response status set by RedirectView.

Forwarding

It is also possible to use a special forward: prefix for view names that are
ultimately resolved by UrlBasedViewResolver and subclasses. This creates an
InternalResourceView which does a RequestDispatcher.forward().
Therefore, this prefix is not useful with InternalResourceViewResolver and
InternalResourceView (for JSPs) but it can be helpful if using another view
technology, but still want to force a forward of a resource to be handled by the
Servlet/JSP engine. Note that you may also chain multiple view resolvers, instead.

Content negotiation

ContentNegotiatingViewResolver
does not resolve views itself but rather delegates
to other view resolvers, and selects the view that resembles the representation requested
by the client. The representation can be determined from the Accept header or from a
query parameter, e.g. "/path?format=pdf".

The ContentNegotiatingViewResolver selects an appropriate View to handle the request
by comparing the request media type(s) with the media type (also known as
Content-Type) supported by the View associated with each of its ViewResolvers. The
first View in the list that has a compatible Content-Type returns the representation
to the client. If a compatible view cannot be supplied by the ViewResolver chain, then
the list of views specified through the DefaultViews property will be consulted. This
latter option is appropriate for singleton Views that can render an appropriate
representation of the current resource regardless of the logical view name. The Accept
header may include wild cards, for example text/*, in which case a View whose
Content-Type was text/xml is a compatible match.

1.2.9. Locale

Most parts of Spring’s architecture support internationalization, just as the Spring web
MVC framework does. DispatcherServlet enables you to automatically resolve messages
using the client’s locale. This is done with LocaleResolver objects.

When a request comes in, the DispatcherServlet looks for a locale resolver, and if it
finds one it tries to use it to set the locale. Using the RequestContext.getLocale()
method, you can always retrieve the locale that was resolved by the locale resolver.

In addition to automatic locale resolution, you can also attach an interceptor to the
handler mapping (see Interception for more information on handler
mapping interceptors) to change the locale under specific circumstances, for example,
based on a parameter in the request.

Locale resolvers and interceptors are defined in the
org.springframework.web.servlet.i18n package and are configured in your application
context in the normal way. Here is a selection of the locale resolvers included in
Spring.

TimeZone

In addition to obtaining the client’s locale, it is often useful to know their time zone.
The LocaleContextResolver interface offers an extension to LocaleResolver that allows
resolvers to provide a richer LocaleContext, which may include time zone information.

When available, the user’s TimeZone can be obtained using the
RequestContext.getTimeZone() method. Time zone information will automatically be used
by Date/Time Converter and Formatter objects registered with Spring’s
ConversionService.

Header resolver

This locale resolver inspects the accept-language header in the request that was sent
by the client (e.g., a web browser). Usually this header field contains the locale of
the client’s operating system. Note that this resolver does not support time zone
information.

Cookie resolver

This locale resolver inspects a Cookie that might exist on the client to see if a
Locale or TimeZone is specified. If so, it uses the specified details. Using the
properties of this locale resolver, you can specify the name of the cookie as well as the
maximum age. Find below an example of defining a CookieLocaleResolver.

<beanid="localeResolver"class="org.springframework.web.servlet.i18n.CookieLocaleResolver"><propertyname="cookieName"value="clientlanguage"/><!-- in seconds. If set to -1, the cookie is not persisted (deleted when browser shuts down) --><propertyname="cookieMaxAge"value="100000"/></bean>

Table 4. CookieLocaleResolver properties

Property

Default

Description

cookieName

classname + LOCALE

The name of the cookie

cookieMaxAge

Servlet container default

The maximum time a cookie will stay persistent on the client. If -1 is specified, the
cookie will not be persisted; it will only be available until the client shuts down
their browser.

cookiePath

/

Limits the visibility of the cookie to a certain part of your site. When cookiePath is
specified, the cookie will only be visible to that path and the paths below it.

Session resolver

The SessionLocaleResolver allows you to retrieve Locale and TimeZone from the
session that might be associated with the user’s request. In contrast to
CookieLocaleResolver, this strategy stores locally chosen locale settings in the
Servlet container’s HttpSession. As a consequence, those settings are just temporary
for each session and therefore lost when each session terminates.

Note that there is no direct relationship with external session management mechanisms
such as the Spring Session project. This SessionLocaleResolver will simply evaluate and
modify corresponding HttpSession attributes against the current HttpServletRequest.

Locale interceptor

You can enable changing of locales by adding the LocaleChangeInterceptor to one of the
handler mappings (see [mvc-handlermapping]). It will detect a parameter in the request
and change the locale. It calls setLocale() on the LocaleResolver that also exists
in the context. The following example shows that calls to all *.view resources
containing a parameter named siteLanguage will now change the locale. So, for example,
a request for the following URL, http://www.sf.net/home.view?siteLanguage=nl will
change the site language to Dutch.

1.2.10. Themes

You can apply Spring Web MVC framework themes to set the overall look-and-feel of your
application, thereby enhancing user experience. A theme is a collection of static
resources, typically style sheets and images, that affect the visual style of the
application.

Define a theme

To use themes in your web application, you must set up an implementation of the
org.springframework.ui.context.ThemeSource interface. The WebApplicationContext
interface extends ThemeSource but delegates its responsibilities to a dedicated
implementation. By default the delegate will be an
org.springframework.ui.context.support.ResourceBundleThemeSource implementation that
loads properties files from the root of the classpath. To use a custom ThemeSource
implementation or to configure the base name prefix of the ResourceBundleThemeSource,
you can register a bean in the application context with the reserved name themeSource.
The web application context automatically detects a bean with that name and uses it.

When using the ResourceBundleThemeSource, a theme is defined in a simple properties
file. The properties file lists the resources that make up the theme. Here is an example:

The keys of the properties are the names that refer to the themed elements from view
code. For a JSP, you typically do this using the spring:theme custom tag, which is
very similar to the spring:message tag. The following JSP fragment uses the theme
defined in the previous example to customize the look and feel:

By default, the ResourceBundleThemeSource uses an empty base name prefix. As a result,
the properties files are loaded from the root of the classpath. Thus you would put the
cool.properties theme definition in a directory at the root of the classpath, for
example, in /WEB-INF/classes. The ResourceBundleThemeSource uses the standard Java
resource bundle loading mechanism, allowing for full internationalization of themes. For
example, we could have a /WEB-INF/classes/cool_nl.properties that references a special
background image with Dutch text on it.

Resolve themes

After you define themes, as in the preceding section, you decide which theme to use. The
DispatcherServlet will look for a bean named themeResolver to find out which
ThemeResolver implementation to use. A theme resolver works in much the same way as a
LocaleResolver. It detects the theme to use for a particular request and can also
alter the request’s theme. The following theme resolvers are provided by Spring:

Table 5. ThemeResolver implementations

Class

Description

FixedThemeResolver

Selects a fixed theme, set using the defaultThemeName property.

SessionThemeResolver

The theme is maintained in the user’s HTTP session. It only needs to be set once for
each session, but is not persisted between sessions.

CookieThemeResolver

The selected theme is stored in a cookie on the client.

Spring also provides a ThemeChangeInterceptor that allows theme changes on every
request with a simple request parameter.

1.2.11. Multipart resolver

MultipartResolver from the org.springframework.web.multipart package is a strategy
for parsing multipart requests including file uploads. There is one implementation
based on Commons FileUpload and another
based on Servlet 3.0 multipart request parsing.

To enable multipart handling, you need declare a MultipartResolver bean in your
DispatcherServlet Spring configuration with the name "multipartResolver".
The DispatcherServlet detects it and applies it to incoming request. When a POST with
content-type of "multipart/form-data" is received, the resolver parses the content and
wraps the current HttpServletRequest as MultipartHttpServletRequest in order to
provide access to resolved parts in addition to exposing them as request parameters.

Apache FileUpload

To use Apache Commons FileUpload, simply configure a bean of type
CommonsMultipartResolver with the name multipartResolver. Of course you also need to
have commons-fileupload as a dependency on your classpath.

1.3. Filters

1.3.1. HTTP PUT Form

Browsers can only submit form data via HTTP GET or HTTP POST but non-browser clients can also
use HTTP PUT and PATCH. The Servlet API requires ServletRequest.getParameter*()
methods to support form field access only for HTTP POST.

The spring-web module provides HttpPutFormContentFilter that intercepts HTTP PUT and
PATCH requests with content type application/x-www-form-urlencoded, reads the form data from
the body of the request, and wraps the ServletRequest in order to make the form data
available through the ServletRequest.getParameter*() family of methods.

1.3.2. Forwarded Headers

As a request goes through proxies such as load balancers the host, port, and
scheme may change presenting a challenge for applications that need to create links
to resources since the links should reflect the host, port, and scheme of the
original request as seen from a client perspective.

RFC 7239 defines the "Forwarded" HTTP header
for proxies to use to provide information about the original request. There are also
other non-standard headers in use such as "X-Forwarded-Host", "X-Forwarded-Port",
and "X-Forwarded-Proto".

ForwardedHeaderFilter detects, extracts, and uses information from the "Forwarded"
header, or from "X-Forwarded-Host", "X-Forwarded-Port", and "X-Forwarded-Proto".
It wraps the request in order to overlay its host, port, and scheme and also "hides"
the forwarded headers for subsequent processing.

Note that there are security considerations when using forwarded headers as explained
in Section 8 of RFC 7239. At the application level it is difficult to determine whether
forwarded headers can be trusted or not. This is why the network upstream should be
configured correctly to filter out untrusted forwarded headers from the outside.

Applications that don’t have a proxy and don’t need to use forwarded headers can
configure the ForwardedHeaderFilter to remove and ignore such headers.

1.3.3. Shallow ETag

There is a ShallowEtagHeaderFilter. It is called shallow because it doesn’t have any
knowledge of the content. Instead it relies on buffering actual content written to the
response and computing the ETag value at the end.

1.3.4. CORS

Spring MVC provides fine-grained support for CORS configuration through annotations on
controllers. However when used with Spring Security it is advisable to rely on the built-in
CorsFilter that must be ordered ahead of Spring Security’s chain of filters.

1.4.1. Declaration

You can define controller beans using a standard Spring bean definition in the
Servlet’s WebApplicationContext. The @Controller stereotype allows for auto-detection,
aligned with Spring general support for detecting @Component classes in the classpath
and auto-registering bean definitions for them. It also acts as a stereotype for the
annotated class, indicating its role as a web component.

To enable auto-detection of such @Controller beans, you can add component scanning to
your Java configuration:

@RestController is a composed annotation that is
itself meta-annotated with @Controller and @ResponseBody indicating a controller whose
every method inherits the type-level @ResponseBody annotation and therefore writes
directly to the response body vs view resolution and rendering with an HTML template.

AOP proxies

In some cases a controller may need to be decorated with an AOP proxy at runtime.
One example is if you choose to have @Transactional annotations directly on the
controller. When this is the case, for controllers specifically, we recommend
using class-based proxying. This is typically the default choice with controllers.
However if a controller must implement an interface that is not a Spring Context
callback (e.g. InitializingBean, *Aware, etc), you may need to explicitly
configure class-based proxying. For example with <tx:annotation-driven/>,
change to <tx:annotation-driven proxy-target-class="true"/>.

1.4.2. Request Mapping

The @RequestMapping annotation is used to map requests to controllers methods. It has
various attributes to match by URL, HTTP method, request parameters, headers, and media
types. It can be used at the class-level to express shared mappings or at the method level
to narrow down to a specific endpoint mapping.

There are also HTTP method specific shortcut variants of @RequestMapping:

@GetMapping

@PostMapping

@PutMapping

@DeleteMapping

@PatchMapping

The above are Custom Annotations that are provided out of the box
because arguably most controller methods should be mapped to a specific HTTP method vs
using @RequestMapping which by default matches to all HTTP methods. At the same an
@RequestMapping is still needed at the class level to express shared mappings.

URI variables are automatically converted to the appropriate type or`TypeMismatchException`
is raised. Simple types — int, long, Date, are supported by default and you can
register support for any other data type.
See Type Conversion and DataBinder.

URI variables can be named explicitly — e.g. @PathVariable("customId"), but you can
leave that detail out if the names are the same and your code is compiled with debugging
information or with the -parameters compiler flag on Java 8.

The syntax {varName:regex} declares a URI variable with a regular expressions with the
syntax {varName:regex} — e.g. given URL "/spring-web-3.0.5 .jar", the below method
extracts the name, version, and file extension:

URI path patterns can also have embedded ${…​} placeholders that are resolved on startup
via PropertyPlaceHolderConfigurer against local, system, environment, and other property
sources. This can be used for example to parameterize a base URL based on some external
configuration.

Spring MVC uses the PathMatcher contract and the AntPathMatcher implementation from
spring-core for URI path matching.

Pattern comparison

When multiple patterns match a URL, they must be compared to find the best match. This done
via AntPathMatcher.getPatternComparator(String path) which looks for patterns that more
specific.

A pattern is less specific if it has a lower count of URI variables and single wildcards
counted as 1 and double wildcards counted as 2. Given an equal score, the longer pattern is
chosen. Given the same score and length, the pattern with more URI variables than wildcards
is chosen.

The default mapping pattern /** is excluded from scoring and always
sorted last. Also prefix patterns such as /public/** are considered less
specific than other pattern that don’t have double wildcards.

For the full details see AntPatternComparator in AntPathMatcher and also keep mind that
the PathMatcher implementation used can be customized. See Path Matching
in the configuration section.

Suffix match

By default Spring MVC performs ".*" suffix pattern matching so that a
controller mapped to /person is also implicitly mapped to /person.*.
The file extension is then used to interpret the requested content type to use for
the response (i.e. instead of the "Accept" header), e.g. /person.pdf,
/person.xml, etc.

Using file extensions like this was necessary when browsers used to send Accept headers
that were hard to interpret consistently. At present that is no longer a necessity and
using the "Accept" header should be the preferred choice.

Over time the use of file name extensions has proven problematic in a variety of ways.
It can cause ambiguity when overlayed with the use of URI variables, path parameters,
URI encoding, and it also makes it difficult to reason about URL-based authorization
and security (see next section for more details).

To completely disable the use of file extensions, you must set both of these:

URL-based content negotiation can still be useful, for example when typing a URL in a
browser. To enable that we recommend a query parameter based strategy to avoid most of
the issues that come with file extensions. Or if you must use file extensions, consider
restricting them to a list of explicitly registered extensions through the
mediaTypes property of ContentNeogiationConfigurer.

Suffix match and RFD

Reflected file download (RFD) attack is similar to XSS in that it relies on request input,
e.g. query parameter, URI variable, being reflected in the response. However instead of
inserting JavaScript into HTML, an RFD attack relies on the browser switching to perform a
download and treating the response as an executable script when double-clicked later.

In Spring MVC @ResponseBody and ResponseEntity methods are at risk because
they can render different content types which clients can request via URL path extensions.
Disabling suffix pattern matching and the use of path extensions for content negotiation
lower the risk but are not sufficient to prevent RFD attacks.

To prevent RFD attacks, prior to rendering the response body Spring MVC adds a
Content-Disposition:inline;filename=f.txt header to suggest a fixed and safe download
file. This is done only if the URL path contains a file extension that is neither whitelisted
nor explicitly registered for content negotiation purposes. However it may potentially have
side effects when URLs are typed directly into a browser.

Many common path extensions are whitelisted by default. Applications with custom
HttpMessageConverter implementations can explicitly register file extensions for content
negotiation to avoid having a Content-Disposition header added for those extensions.
See Content Types.

You can declare a shared consumes attribute at the class level. Unlike most other request
mapping attributes however when used at the class level, a method-level consumes attribute
will overrides rather than extend the class level declaration.

The media type can specify a character set. Negated expressions are supported — e.g.
!text/plain means any content type other than "text/plain".

For JSON content type, the UTF-8 charset should be specified even if
RFC7159
clearly states that "no charset parameter is defined for this registration" because some
browsers require it for interpreting correctly UTF-8 special characters.

You can declare a shared produces attribute at the class level. Unlike most other request
mapping attributes however when used at the class level, a method-level produces attribute
will overrides rather than extend the class level declaration.

Parameters, headers

You can narrow request mappings based on request parameter conditions. You can test for the
presence of a request parameter ("myParam"), for the absence ("!myParam"), or for a
specific value ("myParam=myValue"):

HTTP HEAD, OPTIONS

@GetMapping — and also @RequestMapping(method=HttpMethod.GET), support HTTP HEAD
transparently for request mapping purposes. Controller methods don’t need to change.
A response wrapper, applied in javax.servlet.http.HttpServlet, ensures a "Content-Length"
header is set to the number of bytes written and without actually writing to the response.

@GetMapping — and also @RequestMapping(method=HttpMethod.GET), are implicitly mapped to
and also support HTTP HEAD. An HTTP HEAD request is processed as if it were HTTP GET except
but instead of writing the body, the number of bytes are counted and the "Content-Length"
header set.

By default HTTP OPTIONS is handled by setting the "Allow" response header to the list of HTTP
methods listed in all @RequestMapping methods with matching URL patterns.

For a @RequestMapping without HTTP method declarations, the "Allow" header is set to
"GET,HEAD,POST,PUT,PATCH,DELETE,OPTIONS". Controller methods should always declare the
supported HTTP methods for example by using the HTTP method specific variants — @GetMapping, @PostMapping, etc.

@RequestMapping method can be explicitly mapped to HTTP HEAD and HTTP OPTIONS, but that
is not necessary in the common case.

Custom Annotations

Spring MVC supports the use of composed annotations
for request mapping. Those are annotations that are themselves meta-annotated with
@RequestMapping and composed to redeclare a subset (or all) of the @RequestMapping
attributes with a narrower, more specific purpose.

@GetMapping, @PostMapping, @PutMapping, @DeleteMapping, and @PatchMapping are
examples of composed annotations. They’re provided out of the box because arguably most
controller methods should be mapped to a specific HTTP method vs using @RequestMapping
which by default matches to all HTTP methods. If you need an example of composed
annotations, look at how those are declared.

Spring MVC also supports custom request mapping attributes with custom request matching
logic. This is a more advanced option that requires sub-classing
RequestMappingHandlerMapping and overriding the getCustomMethodCondition method where
you can check the custom attribute and return your own RequestCondition.

Method Arguments

The table below shows supported controller method arguments. Reactive types are not supported
for any arguments.

JDK 8’s java.util.Optional is supported as a method argument in combination with
annotations that have a required attribute — e.g. @RequestParam, @RequestHeader,
etc, and is equivalent to required=false.

Controller method argument

Description

WebRequest, NativeWebRequest

Generic access to request parameters, request & session attributes, without direct
use of the Servlet API.

Enforces the presence of a session. As a consequence, such an argument is never null.Note: Session access is not thread-safe. Consider setting the
RequestMappingHandlerAdapter's "synchronizeOnSession" flag to "true" if multiple
requests are allowed to access a session concurrently.

javax.servlet.http.PushBuilder

Servlet 4.0 push builder API for programmatic HTTP/2 resource pushes.
Note that per Servlet spec, the injected PushBuilder instance can be null if the client
does not support that HTTP/2 feature.

For access to the model that is used in HTML controllers and exposed to templates as
part of view rendering.

RedirectAttributes

Specify attributes to use in case of a redirect — i.e. to be appended to the query
string, and/or flash attributes to be stored temporarily until the request after redirect.
See Redirect attributes and Flash attributes.

@ModelAttribute

For access to an existing attribute in the model (instantiated if not present) with
data binding and validation applied. See @ModelAttribute as well as
Model and DataBinder.

Note that use of @ModelAttribute is optional, e.g. to set its attributes.
See "Any other argument" further below in this table.

Errors, BindingResult

For access to errors from validation and data binding for a command object
(i.e. @ModelAttribute argument), or errors from the validation of an @RequestBody or
@RequestPart arguments; an Errors, or BindingResult argument must be declared
immediately after the validated method argument.

SessionStatus + class-level @SessionAttributes

For marking form processing complete which triggers cleanup of session attributes
declared through a class-level @SessionAttributes annotation. See
@SessionAttributes for more details.

UriComponentsBuilder

For preparing a URL relative to the current request’s host, port, scheme, context path, and
the literal part of the servlet mapping also taking into account Forwarded and
X-Forwarded-* headers. See URI Links.

@SessionAttribute

For access to any session attribute; in contrast to model attributes stored in the session
as a result of a class-level @SessionAttributes declaration. See
@SessionAttribute for more details.

If a method argument is not matched to any of the above, by default it is resolved as
an @RequestParam if it is a simple type, as determined by
BeanUtils#isSimpleProperty,
or as an @ModelAttribute otherwise.

Return Values

The table below shows supported controller method return values. Reactive types are
supported for all return values, see below for more details.

Controller method return value

Description

@ResponseBody

The return value is converted through HttpMessageConverters and written to the
response. See @ResponseBody.

HttpEntity<B>, ResponseEntity<B>

The return value specifies the full response including HTTP headers and body be converted
through HttpMessageConverters and written to the response.
See ResponseEntity.

HttpHeaders

For returning a response with headers and no body.

String

A view name to be resolved with ViewResolver's and used together with the implicit
model — determined through command objects and @ModelAttribute methods. The handler
method may also programmatically enrich the model by declaring a Model argument
(see above).

View

A View instance to use for rendering together with the implicit model — determined
through command objects and @ModelAttribute methods. The handler method may also
programmatically enrich the model by declaring a Model argument (see above).

java.util.Map, org.springframework.ui.Model

Attributes to be added to the implicit model with the view name implicitly determined
through a RequestToViewNameTranslator.

@ModelAttribute

An attribute to be added to the model with the view name implicitly determined through
a RequestToViewNameTranslator.

Note that @ModelAttribute is optional. See "Any other return value" further below in
this table.

ModelAndView object

The view and model attributes to use, and optionally a response status.

void

A method with a void return type (or null return value) is considered to have fully
handled the response if it also has a ServletResponse, or an OutputStream argument, or
an @ResponseStatus annotation. The same is true also if the controller has made a positive
ETag or lastModified timestamp check (see @Controller caching for details).

If none of the above is true, a void return type may also indicate "no response body" for
REST controllers, or default view name selection for HTML controllers.

DeferredResult<V>

Produce any of the above return values asynchronously from any thread — e.g. possibly as a
result of some event or callback. See Async Requests and
DeferredResult.

Callable<V>

Produce any of the above return values asynchronously in a Spring MVC managed thread.
See Async Requests and Callable.

Alternative to `DeferredResult with multi-value streams (e.g. Flux, Observable)
collected to a List.

For streaming scenarios — e.g. text/event-stream, application/json+stream — SseEmitter and ResponseBodyEmitter are used instead, where ServletOutputStream
blocking I/O is performed on a Spring MVC managed thread and back pressure applied
against the completion of each write.

If a return value is not matched to any of the above, by default it is treated as a view
name, if it is String or void (default view name selection via
RequestToViewNameTranslator applies); or as a model attribute to be added to the model,
unless it is a simple type, as determined by
BeanUtils#isSimpleProperty
in which case it remains unresolved.

Type Conversion

Some annotated controller method arguments that represent String-based request input — e.g.
@RequestParam, @RequestHeader, @PathVariable, @MatrixVariable, and @CookieValue,
may require type conversion if the argument is declared as something other than String.

For such cases type conversion is automatically applied based on the configured converters.
By default simple types such as int, long, Date, etc. are supported. Type conversion
can be customized through a WebDataBinder, see DataBinder, or by registering
Formatters with the FormattingConversionService, see
Spring Field Formatting.

Matrix variables

RFC 3986 discusses name-value pairs in
path segments. In Spring MVC we refer to those as "matrix variables" based on an
"old post" by Tim Berners-Lee but they
can be also be referred to as URI path parameters.

Matrix variables can appear in any path segment, each variable separated by semicolon and
multiple values separated by comma, e.g. "/cars;color=red,green;year=2012". Multiple
values can also be specified through repeated variable names, e.g.
"color=red;color=green;color=blue".

If a URL is expected to contain matrix variables, the request mapping for a controller
method must use a URI variable to mask that variable content and ensure the request can
be matched successfully independent of matrix variable order and presence.
Below is an example:

Note that you need to enable the use of matrix variables. In the MVC Java config you need
to set a UrlPathHelper with removeSemicolonContent=false via
Path Matching. In the MVC XML namespace, use
<mvc:annotation-driven enable-matrix-variables="true"/>.

Method parameters using this annotation are required by default, but you can specify that
a method parameter is optional by setting @RequestParam's required flag to false
or by declaring the argument with an java.util.Optional wrapper.

Type conversion is applied automatically if the target method parameter type is not
String. See Type Conversion.

When an @RequestParam annotation is declared as Map<String, String> or
MultiValueMap<String, String> argument, the map is populated with all request
parameters.

Note that use of @RequestParam is optional, e.g. to set its attributes.
By default any argument that is a simple value type, as determined by
BeanUtils#isSimpleProperty,
and is not resolved by any other argument resolver, is treated as if it was annotated
with @RequestParam.

Type conversion is applied automatically if the target method parameter type is not
String. See Type Conversion.

When an @RequestHeader annotation is used on a Map<String, String>,
MultiValueMap<String, String>, or HttpHeaders argument, the map is populated
with all header values.

Built-in support is available for converting a comma-separated string into an
array/collection of strings or other types known to the type conversion system. For
example a method parameter annotated with @RequestHeader("Accept") may be of type
String but also String[] or List<String>.

@ModelAttribute

Use the @ModelAttribute annotation on a method argument to access an attribute from the
model, or have it instantiated if not present. The model attribute is also overlaid with
values from HTTP Servlet request parameters whose names match to field names. This is
referred to as data binding and it saves you from having to deal with parsing and
converting individual query parameters and form fields. For example:

From the invocation of a "primary constructor" with arguments matching to Servlet
request parameters; argument names are determined via JavaBeans
@ConstructorProperties or via runtime-retained parameter names in the bytecode.

While it is common to use a Model to populate the model with
attributes, one other alternative is to rely on a Converter<String, T> in combination
with a URI path variable convention. In the example below the model attribute name
"account" matches the URI path variable "account" and the Account is loaded by passing
the String account number through a registered Converter<String, Account>:

After the model attribute instance is obtained, data binding is applied. The
WebDataBinder class matches Servlet request parameter names (query parameters and form
fields) to field names on the target Object. Matching fields are populated after type
conversion is applied where necessary. For more on data binding (and validation) see
Validation. For more on customizing data binding see
DataBinder.

Data binding may result in errors. By default a BindException is raised but to check
for such errors in the controller method, add a BindingResult argument immediately next
to the @ModelAttribute as shown below:

In some cases you may want access to a model attribute without data binding. For such
cases you can inject the Model into the controller and access it directly or
alternatively set @ModelAttribute(binding=false) as shown below:

Validation can be applied automatically after data binding by adding the
javax.validation.Valid annotation or Spring’s @Validated annotation (also see
Bean validation and
Spring validation). For example:

Note that use of @ModelAttribute is optional, e.g. to set its attributes.
By default any argument that is not a simple value type, as determined by
BeanUtils#isSimpleProperty,
and is not resolved by any other argument resolver, is treated as if it was annotated
with @ModelAttribute.

@SessionAttributes

@SessionAttributes is used to store model attributes in the HTTP Servlet session between
requests. It is a type-level annotation that declares session attributes used by a
specific controller. This will typically list the names of model attributes or types of
model attributes which should be transparently stored in the session for subsequent
requests to access.

For example:

@Controller@SessionAttributes("pet")publicclassEditPetForm {
// ...
}

On the first request when a model attribute with the name "pet" is added to the model,
it is automatically promoted to and saved in the HTTP Servlet session. It remains there
until another controller method uses a SessionStatus method argument to clear the
storage:

@SessionAttribute

If you need access to pre-existing session attributes that are managed globally,
i.e. outside the controller (e.g. by a filter), and may or may not be present
use the @SessionAttribute annotation on a method parameter:

Redirect attributes

By default all model attributes are considered to be exposed as URI template variables in
the redirect URL. Of the remaining attributes those that are primitive types or
collections/arrays of primitive types are automatically appended as query parameters.

Appending primitive type attributes as query parameters may be the desired result if a
model instance was prepared specifically for the redirect. However, in annotated
controllers the model may contain additional attributes added for rendering purposes (e.g.
drop-down field values). To avoid the possibility of having such attributes appear in the
URL, an @RequestMapping method can declare an argument of type RedirectAttributes and
use it to specify the exact attributes to make available to RedirectView. If the method
does redirect, the content of RedirectAttributes is used. Otherwise the content of the
model is used.

The RequestMappingHandlerAdapter provides a flag called
"ignoreDefaultModelOnRedirect" that can be used to indicate the content of the default
Model should never be used if a controller method redirects. Instead the controller
method should declare an attribute of type RedirectAttributes or if it doesn’t do so
no attributes should be passed on to RedirectView. Both the MVC namespace and the MVC
Java config keep this flag set to false in order to maintain backwards compatibility.
However, for new applications we recommend setting it to true

Note that URI template variables from the present request are automatically made
available when expanding a redirect URL and do not need to be added explicitly neither
through Model nor RedirectAttributes. For example:

Another way of passing data to the redirect target is via Flash Attributes. Unlike
other redirect attributes, flash attributes are saved in the HTTP session (and hence do
not appear in the URL). See Flash attributes for more information.

Flash attributes

Flash attributes provide a way for one request to store attributes intended for use in
another. This is most commonly needed when redirecting — for example, the
Post/Redirect/Get pattern. Flash attributes are saved temporarily before the
redirect (typically in the session) to be made available to the request after the
redirect and removed immediately.

Spring MVC has two main abstractions in support of flash attributes. FlashMap is used
to hold flash attributes while FlashMapManager is used to store, retrieve, and manage
FlashMap instances.

Flash attribute support is always "on" and does not need to enabled explicitly although
if not used, it never causes HTTP session creation. On each request there is an "input"
FlashMap with attributes passed from a previous request (if any) and an "output"
FlashMap with attributes to save for a subsequent request. Both FlashMap instances
are accessible from anywhere in Spring MVC through static methods in
RequestContextUtils.

Annotated controllers typically do not need to work with FlashMap directly. Instead an
@RequestMapping method can accept an argument of type RedirectAttributes and use it
to add flash attributes for a redirect scenario. Flash attributes added via
RedirectAttributes are automatically propagated to the "output" FlashMap. Similarly,
after the redirect, attributes from the "input" FlashMap are automatically added to the
Model of the controller serving the target URL.

Matching requests to flash attributes

The concept of flash attributes exists in many other Web frameworks and has proven to be
exposed sometimes to concurrency issues. This is because by definition flash attributes
are to be stored until the next request. However the very "next" request may not be the
intended recipient but another asynchronous request (e.g. polling or resource requests)
in which case the flash attributes are removed too early.

To reduce the possibility of such issues, RedirectView automatically "stamps"
FlashMap instances with the path and query parameters of the target redirect URL. In
turn the default FlashMapManager matches that information to incoming requests when
looking up the "input" FlashMap.

This does not eliminate the possibility of a concurrency issue entirely but nevertheless
reduces it greatly with information that is already available in the redirect URL.
Therefore the use of flash attributes is recommended mainly for redirect scenarios .

Multipart

After a MultipartResolver has been enabled, the content of POST
requests with "multipart/form-data" is parsed and accessible as regular request
parameters. In the example below we access one regular form field and one uploaded
file:

You can access the "meta-data" part with @RequestParam as a String but you’ll
probably want it deserialized from JSON (similar to @RequestBody). Use the
@RequestPart annotation to access a multipart after converting it with an
HttpMessageConverter:

@RequestPart can be used in combination with javax.validation.Valid, or Spring’s
@Validated annotation, which causes Standard Bean Validation to be applied.
By default validation errors cause a MethodArgumentNotValidException which is turned
into a 400 (BAD_REQUEST) response. Alternatively validation errors can be handled locally
within the controller through an Errors or BindingResult argument:

@RequestBody can be used in combination with javax.validation.Valid, or Spring’s
@Validated annotation, which causes Standard Bean Validation to be applied.
By default validation errors cause a MethodArgumentNotValidException which is turned
into a 400 (BAD_REQUEST) response. Alternatively validation errors can be handled locally
within the controller through an Errors or BindingResult argument:

@ResponseBody is also supported at the class level in which case it is inherited by
all controller methods. This is the effect of @RestController which is nothing more
than a meta-annotation marked with @Controller and @ResponseBody.

Jackson JSON

Jackson serialization views

Spring MVC provides built-in support for
Jackson’s Serialization Views
which allows rendering only a subset of all fields in an Object. To use it with
@ResponseBody or ResponseEntity controller methods, use Jackson’s
@JsonView annotation to activate a serialization view class:

Jackson JSONP

In order to enable JSONP support for @ResponseBody
and ResponseEntity methods, declare an @ControllerAdvice bean that extends
AbstractJsonpResponseBodyAdvice as shown below where the constructor argument indicates
the JSONP query parameter name(s):

For controllers relying on view resolution, JSONP is automatically enabled when the
request has a query parameter named jsonp or callback. Those names can be
customized through jsonpParameterNames property.

1.4.4. Model

On a method argument in @RequestMapping methods
to create or access an Object from the model, and to bind it to the request through a
WebDataBinder.

As a method-level annotation in @Controller or @ControllerAdvice classes helping
to initialize the model prior to any @RequestMapping method invocation.

On a @RequestMapping method to mark its return value is a model attribute.

This section discusses @ModelAttribute methods, or the 2nd from the list above.
A controller can have any number of @ModelAttribute methods. All such methods are
invoked before @RequestMapping methods in the same controller. A @ModelAttribute
method can also be shared across controllers via @ControllerAdvice. See the section on
Controller Advice for more details.

@ModelAttribute methods have flexible method signatures. They support many of the same
arguments as @RequestMapping methods except for @ModelAttribute itself nor anything
related to the request body.

When a name is not explicitly specified, a default name is chosen based on the Object
type as explained in the Javadoc for
Conventions.
You can always assign an explicit name by using the overloaded addAttribute method or
through the name attribute on @ModelAttribute (for a return value).

@ModelAttribute can also be used as a method-level annotation on @RequestMapping
methods in which case the return value of the @RequestMapping method is interpreted as a
model attribute. This is typically not required, as it is the default behavior in HTML
controllers, unless the return value is a String which would otherwise be interpreted
as a view name (also see [mvc-coc-r2vnt]). @ModelAttribute can also help to customize
the model attribute name:

@InitBinder methods can register controller-specific java.bean.PropertyEditor, or
Spring Converter and Formatter components. In addition, the
MVC config can be used to register Converter and Formatter
types in a globally shared FormattingConversionService.

@InitBinder methods support many of the same arguments that a @RequestMapping methods
do, except for @ModelAttribute (command object) arguments. Typically they’re are declared
with a WebDataBinder argument, for registrations, and a void return value.
Below is an example:

The exception may match against a top-level exception being propagated (i.e. a direct
IOException thrown), or against the immediate cause within a top-level wrapper exception
(e.g. an IOException wrapped inside an IllegalStateException).

For matching exception types, preferably declare the target exception as a method argument
as shown above. When multiple exception methods match, a root exception match is generally
preferred to a cause exception match. More specifically, the ExceptionDepthComparator is
used to sort exceptions based on their depth from the thrown exception type.

Alternatively, the annotation declaration may narrow the exception types to match:

The distinction between root and cause exception matching can be surprising:

In the IOException variant above, the method will typically be called with
the actual FileSystemException or RemoteException instance as the argument
since both of them extend from IOException. However, if any such matching
exception is propagated within a wrapper exception which is an IOException
itself, the passed-in exception instance will be that wrapper exception.

The behavior is even simpler in the handle(Exception) variant: This will
always be invoked with the wrapper exception in a wrapping scenario, with the
actually matching exception to be found through ex.getCause() in that case.
The passed-in exception will only be the actual FileSystemException or
RemoteException instance when these are thrown as top-level exceptions.

We generally recommend to be as specific as possible in the argument signature,
reducing the potential for mismatches between root and cause exception types.
Consider breaking a multi-matching method into individual @ExceptionHandler
methods, each matching a single specific exception type through its signature.

In a multi-@ControllerAdvice arrangement, please declare your primary root exception
mappings on a @ControllerAdvice prioritized with a corresponding order. While a root
exception match is preferred to a cause, this is defined among the methods of a given
controller or @ControllerAdvice class. This means a cause match on a higher-priority
@ControllerAdvice bean is preferred to any match (e.g. root) on a lower-priority
@ControllerAdvice bean.

Last but not least, an @ExceptionHandler method implementation may choose to back
out of dealing with a given exception instance by rethrowing it in its original form.
This is useful in scenarios where you are only interested in root-level matches or in
matches within a specific context that cannot be statically determined. A rethrown
exception will be propagated through the remaining resolution chain, just like if
the given @ExceptionHandler method would not have matched in the first place.

Support for @ExceptionHandler methods in Spring MVC is built on the DispatcherServlet
level, HandlerExceptionResolver mechanism.

Method arguments

@ExceptionHandler methods support the following arguments:

Method argument

Description

Exception type

For access to the raised exception.

HandlerMethod

For access to the controller method that raised the exception.

WebRequest, NativeWebRequest

Generic access to request parameters, request & session attributes, without direct
use of the Servlet API.

Enforces the presence of a session. As a consequence, such an argument is never null.Note: Session access is not thread-safe. Consider setting the
RequestMappingHandlerAdapter's "synchronizeOnSession" flag to "true" if multiple
requests are allowed to access a session concurrently.

Specify attributes to use in case of a redirect — i.e. to be appended to the query
string, and/or flash attributes to be stored temporarily until the request after redirect.
See Redirect attributes and Flash attributes.

@SessionAttribute

For access to any session attribute; in contrast to model attributes stored in the session
as a result of a class-level @SessionAttributes declaration. See
@SessionAttribute for more details.

Return Values

The return value is converted through HttpMessageConverters and written to the
response. See @ResponseBody.

HttpEntity<B>, ResponseEntity<B>

The return value specifies the full response including HTTP headers and body be converted
through HttpMessageConverters and written to the response.
See ResponseEntity.

String

A view name to be resolved with ViewResolver's and used together with the implicit
model — determined through command objects and @ModelAttribute methods. The handler
method may also programmatically enrich the model by declaring a Model argument
(see above).

View

A View instance to use for rendering together with the implicit model — determined
through command objects and @ModelAttribute methods. The handler method may also
programmatically enrich the model by declaring a Model argument (see above).

java.util.Map, org.springframework.ui.Model

Attributes to be added to the implicit model with the view name implicitly determined
through a RequestToViewNameTranslator.

@ModelAttribute

An attribute to be added to the model with the view name implicitly determined through
a RequestToViewNameTranslator.

Note that @ModelAttribute is optional. See "Any other return value" further below in
this table.

ModelAndView object

The view and model attributes to use, and optionally a response status.

void

A method with a void return type (or null return value) is considered to have fully
handled the response if it also has a ServletResponse, or an OutputStream argument, or
an @ResponseStatus annotation. The same is true also if the controller has made a positive
ETag or lastModified timestamp check (see @Controller caching for details).

If none of the above is true, a void return type may also indicate "no response body" for
REST controllers, or default view name selection for HTML controllers.

Any other return value

If a return value is not matched to any of the above, by default it is treated as a
model attribute to be added to the model, unless it is a simple type, as determined by
BeanUtils#isSimpleProperty
in which case it remains unresolved.

REST API exceptions

A common requirement for REST services is to include error details in the body of the
response. The Spring Framework does not automatically do this because the representation
of error details in the response body is application specific. However a
@RestController may use @ExceptionHandler methods with a ResponseEntity return
value to set the status and the body of the response. Such methods may also be declared
in @ControllerAdvice classes to apply them globally.

Applications that implement global exception handling with error details in the response
body should consider extending
ResponseEntityExceptionHandler
which provides handling for exceptions that Spring MVC raises along with hooks to
customize the response body. To make use of this, create a subclass of
ResponseEntityExceptionHandler, annotate with @ControllerAdvice, override the
necessary methods, and declare it as a Spring bean.

1.4.7. Controller Advice

Typically @ExceptionHandler, @InitBinder, and @ModelAttribute methods apply within
the @Controller class (or class hierarchy) they are declared in. If you want such
methods to apply more globally, across controllers, you can declare them in a class
marked with @ControllerAdvice or @RestControllerAdvice.

@ControllerAdvice is marked with @Component which means such classes can be registered
as Spring beans via component scanning.
@RestControllerAdvice is also a meta-annotation marked with both @ControllerAdvice and
@ResponseBody which essentially means @ExceptionHandler methods are rendered to the
response body via message conversion (vs view resolution/template rendering).

On startup, the infrastructure classes for @RequestMapping and @ExceptionHandler methods
detect Spring beans of type @ControllerAdvice, and then apply their methods at runtime.
Global @ExceptionHandler methods (from an @ControllerAdvice) are applied after local
ones (from the @Controller). By contrast global @ModelAttribute and @InitBinder
methods are applied before local ones.

By default @ControllerAdvice methods apply to every request, i.e. all controllers, but
you can narrow that down to a subset of controllers via attributes on the annotation:

1.5.2. UriBuilder

Spring MVC and Spring WebFlux

UriComponentsBuilder is an implementation of UriBuilder. Together
UriBuilderFactory and UriBuilder provide a pluggable mechanism for building a URI
from a URI template, as well as a way to share common properties such as a base URI,
encoding strategy, and others.

Both the RestTemplate and the WebClient can be configured with a UriBuilderFactory
in order to customize how URIs are created from URI templates. The default implementation
relies on UriComponentsBuilder internally and provides options to configure a common
base URI, an alternative encoding mode strategy, and more.

You can also use DefaultUriBuilderFactory directly, as you would UriComponentsBuilder.
The main difference is that DefaultUriBuilderFactory is stateful and can be re-used to
prepare many URLs, sharing common configuration, such as a base URL, while
UriComponentsBuilder is stateless and per URI.

1.5.3. URI Encoding

Spring MVC and Spring WebFlux

The default way of encoding URIs in UriComponents works as follows:

URI variables are expanded.

Each URI component (path, query, etc) is encoded individually.

The rules for encoding are as follows: within a URI component, apply percent encoding to
all illegal characters, including non-US-ASCII characters, and all other characters that
are illegal within the URI component, as defined in RFC 3986.

The encoding in UriComponents is comparable to the multi-argument constructor of
java.net.URI, as described in the "Escaped octets, quotation, encoding, and decoding"
section of its class-level Javadoc.

The above default encoding strategy does not encode all characters with reserved
meaning, but only the ones that are illegal within a given URI component. If this is not
what you expect, you can use the alternative strategy described next.

When using DefaultUriBuilderFactory — plugged into the WebClient,
RestTemplate, or used directly, you can switch to an alternative encoding strategy as
shown below:

This alternative encoding strategy applies UriUtils.encode(String, Charset) to each URI
variable value prior to expanding it, effectively encoding all non-US-ASCII characters,
and all characters with reserved meaning in a URI, which ensures that the expanded URI
variables do not have any impact on the structure or meaning of the URI.

1.5.4. Servlet request relative

You can use ServletUriComponentsBuilder to create URIs relative to the current request:

ServletUriComponentsBuilder detects and uses information from the "Forwarded",
"X-Forwarded-Host", "X-Forwarded-Port", and "X-Forwarded-Proto" headers, so the resulting
links reflect the original request. You need to ensure that your application is behind
a trusted proxy which filters out such headers coming from outside. Also consider using
the ForwardedHeaderFilter which processes such headers once
per request, and also provides an option to remove and ignore such headers.

1.5.5. Links to controllers

Spring MVC provides a mechanism to prepare links to controller methods. For example,
the following MVC controller easily allows for link creation:

In the above example we provided actual method argument values, in this case the long value 21,
to be used as a path variable and inserted into the URL. Furthermore, we provided the
value 42 in order to fill in any remaining URI variables such as the "hotel" variable inherited
from the type-level request mapping. If the method had more arguments you can supply null for
arguments not needed for the URL. In general only @PathVariable and @RequestParam arguments
are relevant for constructing the URL.

There are additional ways to use MvcUriComponentsBuilder. For example you can use a technique
akin to mock testing through proxies to avoid referring to the controller method by name
(the example assumes static import of MvcUriComponentsBuilder.on):

Controller method signatures are limited in their design when supposed to be usable for
link creation with fromMethodCall. Aside from needing a proper parameter signature,
there is a technical limitation on the return type: namely generating a runtime proxy
for link builder invocations, so the return type must not be final. In particular,
the common String return type for view names does not work here; use ModelAndView
or even plain Object (with a String return value) instead.

The above examples use static methods in MvcUriComponentsBuilder. Internally they rely
on ServletUriComponentsBuilder to prepare a base URL from the scheme, host, port,
context path and servlet path of the current request. This works well in most cases,
however sometimes it may be insufficient. For example you may be outside the context of
a request (e.g. a batch process that prepares links) or perhaps you need to insert a path
prefix (e.g. a locale prefix that was removed from the request path and needs to be
re-inserted into links).

For such cases you can use the static "fromXxx" overloaded methods that accept a
UriComponentsBuilder to use base URL. Or you can create an instance of MvcUriComponentsBuilder
with a base URL and then use the instance-based "withXxx" methods. For example:

MvcUriComponentsBuilder detects and uses information from the "Forwarded",
"X-Forwarded-Host", "X-Forwarded-Port", and "X-Forwarded-Proto" headers, so the resulting
links reflect the original request. You need to ensure that your application is behind
a trusted proxy which filters out such headers coming from outside. Also consider using
the ForwardedHeaderFilter which processes such headers once
per request, and also provides an option to remove and ignore such headers.

1.5.6. Links in views

You can also build links to annotated controllers from views such as JSP, Thymeleaf,
FreeMarker. This can be done using the fromMappingName method in MvcUriComponentsBuilder
which refers to mappings by name.

Every @RequestMapping is assigned a default name based on the capital letters of the
class and the full method name. For example, the method getFoo in class FooController
is assigned the name "FC#getFoo". This strategy can be replaced or customized by creating
an instance of HandlerMethodMappingNamingStrategy and plugging it into your
RequestMappingHandlerMapping. The default strategy implementation also looks at the
name attribute on @RequestMapping and uses that if present. That means if the default
mapping name assigned conflicts with another (e.g. overloaded methods) you can assign
a name explicitly on the @RequestMapping.

The assigned request mapping names are logged at TRACE level on startup.

The Spring JSP tag library provides a function called mvcUrl that can be used to
prepare links to controller methods based on this mechanism.

The above example relies on the mvcUrl JSP function declared in the Spring tag library
(i.e. META-INF/spring.tld). For more advanced cases (e.g. a custom base URL as explained
in the previous section), it is easy to define your own function, or use a custom tag file,
in order to use a specific instance of MvcUriComponentsBuilder with a custom base URL.

1.6.3. Processing

Here is a very concise overview of Servlet asynchronous request processing:

A ServletRequest can be put in asynchronous mode by calling request.startAsync().
The main effect of doing so is that the Servlet, as well as any Filters, can exit but
the response will remain open to allow processing to complete later.

The call to request.startAsync() returns AsyncContext which can be used for
further control over async processing. For example it provides the method dispatch,
that is similar to a forward from the Servlet API except it allows an
application to resume request processing on a Servlet container thread.

The ServletRequest provides access to the current DispatcherType that can
be used to distinguish between processing the initial request, an async
dispatch, a forward, and other dispatcher types.

DeferredResult processing:

Controller returns a DeferredResult and saves it in some in-memory
queue or list where it can be accessed.

Spring MVC calls request.startAsync().

Meanwhile the DispatcherServlet and all configured Filter’s exit the request
processing thread but the response remains open.

The application sets the DeferredResult from some thread and Spring MVC
dispatches the request back to the Servlet container.

The DispatcherServlet is invoked again and processing resumes with the
asynchronously produced return value.

Callable processing:

Controller returns a Callable.

Spring MVC calls request.startAsync() and submits the Callable to
a TaskExecutor for processing in a separate thread.

Meanwhile the DispatcherServlet and all Filter’s exit the Servlet container thread
but the response remains open.

Eventually the Callable produces a result and Spring MVC dispatches the request back
to the Servlet container to complete processing.

The DispatcherServlet is invoked again and processing resumes with the
asynchronously produced return value from the Callable.

For further background and context you can also read
the
blog posts that introduced asynchronous request processing support in Spring MVC 3.2.

Exception handling

When using a DeferredResult you can choose whether to call setResult or
setErrorResult with an exception. In both cases Spring MVC dispatches the request back
to the Servlet container to complete processing. It is then treated either as if the
controller method returned the given value, or as if it produced the given exception.
The exception then goes through the regular exception handling mechanism, e.g. invoking
@ExceptionHandler methods.

When using Callable, similar processing logic follows. The main difference being that
the result is returned from the Callable or an exception is raised by it.

Interception

HandlerInterceptor's can also be AsyncHandlerInterceptor in order to receive the
afterConcurrentHandlingStarted callback on the initial request that starts asynchronous
processing instead of postHandle and afterCompletion.

HandlerInterceptor's can also register a CallableProcessingInterceptor
or a DeferredResultProcessingInterceptor in order to integrate more deeply with the
lifecycle of an asynchronous request for example to handle a timeout event. See
AsyncHandlerInterceptor
for more details.

DeferredResult provides onTimeout(Runnable) and onCompletion(Runnable) callbacks.
See the Javadoc of DeferredResult for more details. Callable can be substituted for
WebAsyncTask that exposes additional methods for timeout and completion callbacks.

Compared to WebFlux

The Servlet API was originally built for making a single pass through the Filter-Servlet
chain. Asynchronous request processing, added in Servlet 3.0, allows applications to exit
the Filter-Servlet chain but leave the response open for further processing. The Spring MVC
async support is built around that mechanism. When a controller returns a DeferredResult,
the Filter-Servlet chain is exited and the Servlet container thread is released. Later when
the DeferredResult is set, an ASYNC dispatch (to the same URL) is made during which the
controller is mapped again but rather than invoking it, the DeferredResult value is used
(as if the controller returned it) to resume processing.

By contrast Spring WebFlux is neither built on the Servlet API, nor does it need such an
asynchronous request processing feature because it is asynchronous by design. Asynchronous
handling is built into all framework contracts and is intrinsically supported through ::
stages of request processing.

From a programming model perspective, both Spring MVC and Spring WebFlux support
asynchronous and Reactive types as return values in controller methods.
Spring MVC even supports streaming, including reactive back pressure. However individual
writes to the response remain blocking (and performed on a separate thread) unlike WebFlux
that relies on non-blocking I/O and does not need an extra thread for each write.

Another fundamental difference is that Spring MVC does not support asynchronous or
reactive types in controller method arguments, e.g. @RequestBody, @RequestPart, and
others, nor does it have any explicit support for asynchronous and reactive types as
model attributes. Spring WebFlux does support all that.

1.6.4. HTTP Streaming

DeferredResult and Callable can be used for a single asynchronous return value.
What if you want to produce multiple asynchronous values and have those written to the
response?

Objects

The ResponseBodyEmitter return value can be used to produce a stream of Objects, where
each Object sent is serialized with an
HttpMessageConverter and written to the
response. For example:

@GetMapping("/events")
public ResponseBodyEmitter handle() {
ResponseBodyEmitter emitter = new ResponseBodyEmitter();
// Save the emitter somewhere..return emitter;
}
// In some other thread
emitter.send("Hello once");
// and again later on
emitter.send("Hello again");
// and done at some point
emitter.complete();

ResponseBodyEmitter can also be used as the body in a ResponseEntity allowing you to
customize the status and headers of the response.

When an emitter throws an IOException (e.g. if the remote client went away) applications
are not responsible for cleaning up the connection, and should not invoke emitter.complete
or emitter.completeWithError. Instead the servlet container automatically initiates an
AsyncListener error notification in which Spring MVC makes a completeWithError call,
which in turn performs one a final ASYNC dispatch to the application during which Spring MVC
invokes the configured exception resolvers and completes the request.

SSE

SseEmitter is a sub-class of ResponseBodyEmitter that provides support for
Server-Sent Events where events sent from the server
are formatted according to the W3C SSE specification. In order to produce an SSE
stream from a controller simply return SseEmitter:

@GetMapping(path="/events", produces=MediaType.TEXT_EVENT_STREAM_VALUE)
public SseEmitter handle() {
SseEmitter emitter = new SseEmitter();
// Save the emitter somewhere..return emitter;
}
// In some other thread
emitter.send("Hello once");
// and again later on
emitter.send("Hello again");
// and done at some point
emitter.complete();

While SSE is the main option for streaming into browsers, note that Internet Explorer
does not support Server-Sent Events. Consider using Spring’s
WebSocket messaging with
SockJS fallback transports (including SSE) that target
a wide range of browsers.

1.6.5. Reactive types

Spring MVC supports use of reactive client libraries in a controller. This includes the
WebClient from spring-webflux and others such as Spring Data reactive data
repositories. In such scenarios it is convenient to be able to return reactive types
from the controller method .

Reactive return values are handled as follows:

A single-value promise is adapted to, and similar to using DeferredResult. Examples
include Mono (Reactor) or Single (RxJava).

A multi-value stream, with a streaming media type such as "application/stream+json"
or "text/event-stream", is adapted to, and similar to using ResponseBodyEmitter or
SseEmitter. Examples include Flux (Reactor) or Observable (RxJava).
Applications can also return Flux<ServerSentEvent> or Observable<ServerSentEvent>.

A multi-value stream, with any other media type (e.g. "application/json"), is adapted
to, and similar to using DeferredResult<List<?>>.

Spring MVC supports Reactor and RxJava through the
ReactiveAdapterRegistry from
spring-core which allows it to adapt from multiple reactive libraries.

When streaming to the response via reactive types, Spring MVC supports reactive back
pressure, but still needs to use blocking I/O to perform actual writes. This is done
through the configured MVC TaskExecutor on
a separate thread in order to avoid blocking the upstream source (e.g. a Flux returned
from the WebClient). By default a SyncTaskExecutor is used which is not suitable for
production. SPR-16203 will provide better
defaults in Spring Framework 5.1. In the mean time please configure the executor through
the MVC config.

1.6.6. Disconnects

The Servlet API does not provide any notification when a remote client goes away.
Therefore while streaming to the response, whether via SseEmitter or
<<mvc-ann-async-reactive-types,reactive types>, it is important to send data periodically,
since the write would fail if the client has disconnected. The send could take the form
of an empty (comment-only) SSE event, or any other data that the other side would have to
to interpret as a heartbeat and ignore.

Alternatively consider using web messaging solutions such as
STOMP over WebSocket or WebSocket with SockJS
that have a built-in heartbeat mechanism.

1.6.7. Configuration

The async request processing feature must be enabled at the Servlet container level.
The MVC config also exposes several options for asynchronous requests.

Servlet container

Filter and Servlet declarations have an asyncSupported that needs to be set to true
in order enable asynchronous request processing. In addition, Filter mappings should be
declared to handle the ASYNC javax.servlet.DispatchType.

In Java configuration, when you use AbstractAnnotationConfigDispatcherServletInitializer
to initialize the Servlet container, this is done automatically.

In web.xml configuration, add <async-supported>true</async-supported> to the
DispatcherServlet and to Filter declarations, and also add
<dispatcher>ASYNC</dispatcher> to filter mappings.

Spring MVC

The MVC config exposes options related to async request processing:

Java config — use the configureAsyncSupport callback on WebMvcConfigurer.

XML namespace — use the <async-support> element under <mvc:annotation-driven>.

You can configure the following:

Default timeout value for async requests, which if not set, depends
on the underlying Servlet container (e.g. 10 seconds on Tomcat).

AsyncTaskExecutor to use for blocking writes when streaming with
Reactive types, and also for executing Callable's returned from
controller methods. It is highly recommended to configure this property if you’re
streaming with reactive types or have controller methods that return Callable since
by default it is a SimpleAsyncTaskExecutor.

DeferredResultProcessingInterceptor's and CallableProcessingInterceptor's.

Note that the default timeout value can also be set on a DeferredResult,
ResponseBodyEmitter and SseEmitter. For a Callable, use WebAsyncTask to provide
a timeout value.

1.7. CORS

1.7.1. Introduction

For security reasons browsers prohibit AJAX calls to resources outside the current origin.
For example you could have your bank account in one tab and evil.com in another. Scripts
from evil.com should not be able to make AJAX requests to your bank API with your
credentials, e.g. withdrawing money from your account!

Cross-Origin Resource Sharing (CORS) is a W3C specification
implemented by most browsers that allows you to specify
what kind of cross domain requests are authorized rather than using less secure and less
powerful workarounds based on IFRAME or JSONP.

1.7.2. Processing

The CORS specification distinguishes between preflight, simple, and actual requests.
To learn how CORS works, you can read
this article, among
many others, or refer to the specification for more details.

Spring MVC HandlerMapping's provide built-in support for CORS. After successfully
mapping a request to a handler, HandlerMapping's check the CORS configuration for the
given request and handler and take further actions. Preflight requests are handled
directly while simple and actual CORS requests are intercepted, validated, and have
required CORS response headers set.

In order to enable cross-origin requests (i.e. the Origin header is present and
differs from the host of the request) you need to have some explicitly declared CORS
configuration. If no matching CORS configuration is found, preflight requests are
rejected. No CORS headers are added to the responses of simple and actual CORS requests
and consequently browsers reject them.

Each HandlerMapping can be
configured
individually with URL pattern based CorsConfiguration mappings. In most cases applications
will use the MVC Java config or the XML namespace to declare such mappings, which results
in a single, global map passed to all HadlerMappping's.

Global CORS configuration at the HandlerMapping level can be combined with more
fine-grained, handler-level CORS configuration. For example annotated controllers can use
class or method-level @CrossOrigin annotations (other handlers can implement
CorsConfigurationSource).

The rules for combining global and local configuration are generally additive — e.g.
all global and all local origins. For those attributes where only a single value can be
accepted such as allowCredentials and maxAge, the local overrides the global value. See
CorsConfiguration#combine(CorsConfiguration)
for more details.

allowedCredentials is not enabled by default since that establishes a trust level
that exposes sensitive user-specific information such as cookies and CSRF tokens, and
should only be used where appropriate.

maxAge is set to 30 minutes.

@CrossOrigin is supported at the class level too and inherited by all methods:

1.7.4. Global Config

In addition to fine-grained, controller method level configuration you’ll probably want to
define some global CORS configuration too. You can set URL-based CorsConfiguration
mappings individually on any HandlerMapping. Most applications however will use the
MVC Java config or the MVC XNM namespace to do that.

By default global configuration enables the following:

All origins.

All headers.

GET, HEAD, and POST methods.

allowedCredentials is not enabled by default since that establishes a trust level
that exposes sensitive user-specific information such as cookies and CSRF tokens, and
should only be used where appropriate.

HDIV is another web security framework that integrates with Spring MVC.

1.9. HTTP Caching

A good HTTP caching strategy can significantly improve the performance of a web application
and the experience of its clients. The 'Cache-Control' HTTP response header is mostly
responsible for this, along with conditional headers such as 'Last-Modified' and 'ETag'.

The 'Cache-Control' HTTP response header advises private caches (e.g. browsers) and
public caches (e.g. proxies) on how they can cache HTTP responses for further reuse.

An ETag (entity tag) is an HTTP response header
returned by an HTTP/1.1 compliant web server used to determine change in content at a
given URL. It can be considered to be the more sophisticated successor to the
Last-Modified header. When a server returns a representation with an ETag header, the
client can use this header in subsequent GETs, in an If-None-Match header. If the
content has not changed, the server returns 304: Not Modified.

This section describes the different choices available to configure HTTP caching in a
Spring Web MVC application.

1.9.1. Cache-Control

Spring Web MVC supports many use cases and ways to configure "Cache-Control" headers for
an application. While RFC 7234 Section 5.2.2
completely describes that header and its possible directives, there are several ways to
address the most common cases.

Spring Web MVC uses a configuration convention in several of its APIs:
setCachePeriod(int seconds):

A -1 value won’t generate a 'Cache-Control' response header.

A 0 value will prevent caching using the 'Cache-Control: no-store' directive.

An n > 0 value will cache the given response for n seconds using the
'Cache-Control: max-age=n' directive.

The CacheControl builder
class simply describes the available "Cache-Control" directives and makes it easier to
build your own HTTP caching strategy. Once built, a CacheControl instance can then be
accepted as an argument in several Spring Web MVC APIs.

1.9.2. Static resources

Static resources should be served with appropriate 'Cache-Control' and conditional
headers for optimal performance.
Configuring a ResourceHttpRequestHandler for serving
static resources not only natively writes 'Last-Modified' headers by reading a file’s
metadata, but also 'Cache-Control' headers if properly configured.

You can set the cachePeriod attribute on a ResourceHttpRequestHandler or use
a CacheControl instance, which supports more specific directives:

1.9.3. @Controller caching

Controllers can support 'Cache-Control', 'ETag', and/or 'If-Modified-Since' HTTP requests;
this is indeed recommended if a 'Cache-Control' header is to be set on the response.
This involves calculating a lastModified long and/or an Etag value for a given request,
comparing it against the 'If-Modified-Since' request header value, and potentially returning
a response with status code 304 (Not Modified).

As described in HttpEntity, controllers can interact with the request/response using
HttpEntity types. Controllers returning ResponseEntity can include HTTP caching information
in responses like this:

Doing this will not only include 'ETag' and 'Cache-Control' headers in the response, it will also convert the
response to an HTTP 304 Not Modified response with an empty body if the conditional headers sent by the client
match the caching information set by the Controller.

An @RequestMapping method may also wish to support the same behavior.
This can be achieved as follows:

There are two key elements here: calling request.checkNotModified(lastModified) and
returning null. The former sets the appropriate response status and headers
before it returns true.
The latter, in combination with the former, causes Spring MVC to do no further
processing of the request.

Note that there are 3 variants for this:

request.checkNotModified(lastModified) compares lastModified with the
'If-Modified-Since' or 'If-Unmodified-Since' request header

request.checkNotModified(eTag, lastModified) does both, meaning that both
conditions should be valid

When receiving conditional 'GET'/'HEAD' requests, checkNotModified will check
that the resource has not been modified and if so, it will result in a HTTP 304 Not Modified
response. In case of conditional 'POST'/'PUT'/'DELETE' requests, checkNotModified
will check that the resource has not been modified and if it has been, it will result in a
HTTP 409 Precondition Failed response to prevent concurrent modifications.

1.9.4. ETag Filter

Support for ETags is provided by the Servlet filter ShallowEtagHeaderFilter. It is a
plain Servlet Filter, and thus can be used in combination with any web framework. The
ShallowEtagHeaderFilter filter creates so-called shallow ETags by caching the content
written to the response and generating an MD5 hash over that to send as an ETag header.
The next time a client sends a request for the same resource, it uses that hash as the
If-None-Match value. The filter detects this, lets the request be processed as usual, and
at the end compares the two hashes. If they are equal, a 304 is returned.

Note that this strategy saves network bandwidth but not CPU, as the full response must be
computed for each request. Other strategies at the controller level, described above, can
avoid computation.

This filter has a writeWeakETag parameter that configures the filter to write Weak ETags,
like this: W/"02a2d595e6ed9a0b24f027f2b63b134d6", as defined in
RFC 7232 Section 2.3.

1.10. View Technologies

The use of view technologies in Spring MVC is pluggable, whether you decide to
use Thymeleaf, Groovy Markup Templates, JSPs, or other, is primarily a matter of a
configuration change. This chapter covers view technologies integrated with Spring MVC.
We assume you are already familiar with View Resolution.

1.10.1. Thymeleaf

Thymeleaf is modern server-side Java template engine that emphasizes natural HTML
templates that can be previewed in a browser by double-clicking, which is very
helpful for independent work on UI templates, e.g. by designer, without the need for a
running server. If you’re looking to replace JSPs, Thymeleaf offers one of the most
extensive set of features that will make such a transition easier. Thymeleaf is actively
developed and maintained. For a more complete introduction see the
Thymeleaf project home page.

The Thymeleaf integration with Spring MVC is managed by the Thymeleaf project. The
configuration involves a few bean declarations such as
ServletContextTemplateResolver, SpringTemplateEngine, and ThymeleafViewResolver.
See Thymeleaf+Spring for more details.

1.10.2. FreeMarker

Apache FreeMarker is a template engine for generating any
kind of text output from HTML to email, and others. The Spring Framework has a built-in
integration for using Spring MVC with FreeMarker templates.

Your templates need to be stored in the directory specified by the FreeMarkerConfigurer
shown above. Given the above configuration if your controller returns the view name
"welcome" then the resolver will look for the /WEB-INF/freemarker/welcome.ftl template.

FreeMarker config

FreeMarker 'Settings' and 'SharedVariables' can be passed directly to the FreeMarker
Configuration object managed by Spring by setting the appropriate bean properties on
the FreeMarkerConfigurer bean. The freemarkerSettings property requires a
java.util.Properties object and the freemarkerVariables property requires a
java.util.Map.

See the FreeMarker documentation for details of settings and variables as they apply to
the Configuration object.

Form handling

Spring provides a tag library for use in JSP’s that contains, amongst others, a
<spring:bind/> tag. This tag primarily enables forms to display values from form
backing objects and to show the results of failed validations from a Validator in the
web or business tier. Spring also has support for the same functionality in FreeMarker,
with additional convenience macros for generating form input elements themselves.

The bind macros

A standard set of macros are maintained within the spring-webmvc.jar file for both
languages, so they are always available to a suitably configured application.

Some of the macros defined in the Spring libraries are considered internal (private) but
no such scoping exists in the macro definitions making all macros visible to calling
code and user templates. The following sections concentrate only on the macros you need
to be directly calling from within your templates. If you wish to view the macro code
directly, the file is called spring.ftl in the package
org.springframework.web.servlet.view.freemarker.

Simple binding

In your HTML forms (vm / ftl templates) which act as a form view for a Spring MVC
controller, you can use code similar to the following to bind to field values and
display error messages for each input field in similar fashion to the JSP equivalent.
Example code is shown below for the personForm view configured earlier:

<@spring.bind> requires a 'path' argument which consists of the name of your command
object (it will be 'command' unless you changed it in your FormController properties)
followed by a period and the name of the field on the command object you wish to bind to.
Nested fields can be used too such as "command.address.street". The bind macro assumes
the default HTML escaping behavior specified by the ServletContext parameter
defaultHtmlEscape in web.xml.

The optional form of the macro called <@spring.bindEscaped> takes a second argument
and explicitly specifies whether HTML escaping should be used in the status error
messages or values. Set to true or false as required. Additional form handling macros
simplify the use of HTML escaping and these macros should be used wherever possible.
They are explained in the next section.

Input macros

Additional convenience macros for both languages simplify both binding and form
generation (including validation error display). It is never necessary to use these
macros to generate form input fields, and they can be mixed and matched with simple HTML
or calls direct to the spring bind macros highlighted previously.

The following table of available macros show the FTL definitions and the
parameter list that each takes.

Table 6. Table of macro definitions

macro

FTL definition

message (output a string from a resource bundle based on the code parameter)

<@spring.message code/>

messageText (output a string from a resource bundle based on the code parameter,
falling back to the value of the default parameter)

<@spring.messageText code, text/>

url (prefix a relative URL with the application’s context root)

<@spring.url relativeUrl/>

formInput (standard input field for gathering user input)

<@spring.formInput path, attributes, fieldType/>

formHiddenInput * (hidden input field for submitting non-user input)

<@spring.formHiddenInput path, attributes/>

formPasswordInput * (standard input field for gathering passwords. Note that no
value will ever be populated in fields of this type)

formSingleSelect (drop down box of options allowing a single required value to be
selected)

<@spring.formSingleSelect path, options, attributes/>

formMultiSelect (a list box of options allowing the user to select 0 or more values)

<@spring.formMultiSelect path, options, attributes/>

formRadioButtons (a set of radio buttons allowing a single selection to be made
from the available choices)

<@spring.formRadioButtons path, options separator, attributes/>

formCheckboxes (a set of checkboxes allowing 0 or more values to be selected)

<@spring.formCheckboxes path, options, separator, attributes/>

formCheckbox (a single checkbox)

<@spring.formCheckbox path, attributes/>

showErrors (simplify display of validation errors for the bound field)

<@spring.showErrors separator, classOrStyle/>

In FTL (FreeMarker), formHiddenInput and formPasswordInput are not actually required
as you can use the normal formInput macro, specifying hidden or password as the
value for the fieldType parameter.

The parameters to any of the above macros have consistent meanings:

path: the name of the field to bind to (ie "command.name")

options: a Map of all the available values that can be selected from in the input
field. The keys to the map represent the values that will be POSTed back from the form
and bound to the command object. Map objects stored against the keys are the labels
displayed on the form to the user and may be different from the corresponding values
posted back by the form. Usually such a map is supplied as reference data by the
controller. Any Map implementation can be used depending on required behavior. For
strictly sorted maps, a SortedMap such as a TreeMap with a suitable Comparator may
be used and for arbitrary Maps that should return values in insertion order, use a
LinkedHashMap or a LinkedMap from commons-collections.

separator: where multiple options are available as discreet elements (radio buttons or
checkboxes), the sequence of characters used to separate each one in the list (ie
"<br>").

attributes: an additional string of arbitrary tags or text to be included within the
HTML tag itself. This string is echoed literally by the macro. For example, in a
textarea field you may supply attributes as 'rows="5" cols="60"' or you could pass
style information such as 'style="border:1px solid silver"'.

classOrStyle: for the showErrors macro, the name of the CSS class that the span tag
wrapping each error will use. If no information is supplied (or the value is empty)
then the errors will be wrapped in <b></b> tags.

Examples of the macros are outlined below some in FTL and some in VTL. Where usage
differences exist between the two languages, they are explained in the notes.

Input Fields

The formInput macro takes the path parameter (command.name) and an additional attributes
parameter which is empty in the example above. The macro, along with all other form
generation macros, performs an implicit spring bind on the path parameter. The binding
remains valid until a new bind occurs so the showErrors macro doesn’t need to pass the
path parameter again - it simply operates on whichever field a bind was last created for.

The showErrors macro takes a separator parameter (the characters that will be used to
separate multiple errors on a given field) and also accepts a second parameter, this
time a class name or style attribute. Note that FreeMarker is able to specify default
values for the attributes parameter.

<@spring.formInput "command.name"/><@spring.showErrors "<br>"/>

Output is shown below of the form fragment generating the name field, and displaying a
validation error after the form was submitted with no value in the field. Validation
occurs through Spring’s Validation framework.

The formTextarea macro works the same way as the formInput macro and accepts the same
parameter list. Commonly, the second parameter (attributes) will be used to pass style
information or rows and cols attributes for the textarea.

Selection Fields

Four selection field macros can be used to generate common UI value selection inputs in
your HTML forms.

formSingleSelect

formMultiSelect

formRadioButtons

formCheckboxes

Each of the four macros accepts a Map of options containing the value for the form
field, and the label corresponding to that value. The value and the label can be the
same.

An example of radio buttons in FTL is below. The form backing object specifies a default
value of 'London' for this field and so no validation is necessary. When the form is
rendered, the entire list of cities to choose from is supplied as reference data in the
model under the name 'cityMap'.

This renders a line of radio buttons, one for each value in cityMap using the
separator "". No additional attributes are supplied (the last parameter to the macro is
missing). The cityMap uses the same String for each key-value pair in the map. The map’s
keys are what the form actually submits as POSTed request parameters, map values are the
labels that the user sees. In the example above, given a list of three well known cities
and a default value in the form backing object, the HTML would be

HTML escaping

Default usage of the form macros above will result in HTML tags that are HTML 4.01
compliant and that use the default value for HTML escaping defined in your web.xml as
used by Spring’s bind support. In order to make the tags XHTML compliant or to override
the default HTML escaping value, you can specify two variables in your template (or in
your model where they will be visible to your templates). The advantage of specifying
them in the templates is that they can be changed to different values later in the
template processing to provide different behavior for different fields in your form.

To switch to XHTML compliance for your tags, specify a value of true for a
model/context variable named xhtmlCompliant:

<#-- for FreeMarker -->
<#assign xhtmlCompliant = true>

Any tags generated by the Spring macros will now be XHTML compliant after processing
this directive.

In similar fashion, HTML escaping can be specified per field:

<#-- until this point, default HTML escaping is used -->
<#assign htmlEscape = true>
<#-- next field will use HTML escaping -->
<@spring.formInput "command.name"/>
<#assign htmlEscape = false in spring>
<#-- all future fields will be bound with HTML escaping off -->

1.10.3. Groovy Markup

Groovy Markup Template Engine
is primarily aimed at generating XML-like markup (XML, XHTML, HTML5, etc) but that can
be used to generate any text based content. The Spring Framework has a built-in
integration for using Spring MVC with Groovy Markup.

1.10.4. Script Views

The Spring Framework has a built-in integration for using Spring MVC with any
templating library that can run on top of the
JSR-223 Java scripting engine. Below is a list
of templating libraries we’ve tested on different script engines:

org.jetbrains.kotlin:kotlin-script-util dependency and a META-INF/services/javax.script.ScriptEngineFactory
file containing a org.jetbrains.kotlin.script.jsr223.KotlinJsr223JvmLocalScriptEngineFactory
line should be added for Kotlin script support, see
this example for more details.

You need to have the script templating library. One way to do that for Javascript is
through WebJars.

Script templates

Declare a ScriptTemplateConfigurer bean in order to specify the script engine to use,
the script files to load, what function to call to render templates, and so on.
Below is an example with Mustache templates and the Nashorn JavaScript engine:

RenderingContext renderingContext: the
RenderingContext
that gives access to the application context, the locale, the template loader and the
url (since 5.0)

Mustache.render() is natively compatible with this signature, so you can call it directly.

If your templating technology requires some customization, you may provide a script that
implements a custom render function. For example, Handlerbars
needs to compile templates before using them, and requires a
polyfill in order to emulate some
browser facilities not available in the server-side script engine.

Setting the sharedEngine property to false is required when using non thread-safe
script engines with templating libraries not designed for concurrency, like Handlebars or
React running on Nashorn for example. In that case, Java 8u60 or greater is required due
to this bug.

polyfill.js only defines the window object needed by Handlebars to run properly:

var window = {};

This basic render.js implementation compiles the template before using it. A production
ready implementation should also store and reused cached templates / pre-compiled templates.
This can be done on the script side, as well as any customization you need (managing
template engine configuration for example).

Check out the Spring Framework unit tests,
java, and
resources,
for more configuration examples.

1.10.5. JSP & JSTL

The Spring Framework has a built-in integration for using Spring MVC with JSP and JSTL.

View resolvers

When developing with JSPs you can declare a InternalResourceViewResolver or a
ResourceBundleViewResolver bean.

ResourceBundleViewResolver relies on a properties file to define the view names
mapped to a class and a URL. With a ResourceBundleViewResolver you
can mix different types of views using only one resolver. Here is an example:

InternalResourceBundleViewResolver can also be used for JSPs. As a best practice, we
strongly encourage placing your JSP files in a directory under the 'WEB-INF'
directory so there can be no direct access by clients.

JSPs versus JSTL

When using the Java Standard Tag Library you must use a special view class, the
JstlView, as JSTL needs some preparation before things such as the I18N features will
work.

Spring’s JSP tag library

Spring provides data binding of request parameters to command objects as described in
earlier chapters. To facilitate the development of JSP pages in combination with those
data binding features, Spring provides a few tags that make things even easier. All
Spring tags haveHTML escaping features to enable or disable escaping of characters.

The spring.tld tag library descriptor (TLD) is included in the spring-webmvc.jar.
For a comprehensive reference on individual tags, browse the
API reference
or see the tag library description.

Spring’s form tag library

As of version 2.0, Spring provides a comprehensive set of data binding-aware tags for
handling form elements when using JSP and Spring Web MVC. Each tag provides support for
the set of attributes of its corresponding HTML tag counterpart, making the tags
familiar and intuitive to use. The tag-generated HTML is HTML 4.01/XHTML 1.0 compliant.

Unlike other form/input tag libraries, Spring’s form tag library is integrated with
Spring Web MVC, giving the tags access to the command object and reference data your
controller deals with. As you will see in the following examples, the form tags make
JSPs easier to develop, read and maintain.

Let’s go through the form tags and look at an example of how each tag is used. We have
included generated HTML snippets where certain tags require further commentary.

Configuration

The form tag library comes bundled in spring-webmvc.jar. The library descriptor is
called spring-form.tld.

To use the tags from this library, add the following directive to the top of your JSP
page:

where form is the tag name prefix you want to use for the tags from this library.

The form tag

This tag renders an HTML 'form' tag and exposes a binding path to inner tags for
binding. It puts the command object in the PageContext so that the command object can
be accessed by inner tags. All the other tags in this library are nested tags of the
form tag.

Let’s assume we have a domain object called User. It is a JavaBean with properties
such as firstName and lastName. We will use it as the form backing object of our
form controller which returns form.jsp. Below is an example of what form.jsp would
look like:

The firstName and lastName values are retrieved from the command object placed in
the PageContext by the page controller. Keep reading to see more complex examples of
how inner tags are used with the form tag.

The preceding JSP assumes that the variable name of the form backing object is
'command'. If you have put the form backing object into the model under another name
(definitely a best practice), then you can bind the form to the named variable like so:

The input tag

This tag renders an HTML 'input' tag using the bound value and type='text' by default.
For an example of this tag, see The form tag. Starting with Spring
3.1 you can use other types such HTML5-specific types like 'email', 'tel', 'date', and
others.

The checkbox tag

This tag renders an HTML 'input' tag with type 'checkbox'.

Let’s assume our User has preferences such as newsletter subscription and a list of
hobbies. Below is an example of the Preferences class:

There are 3 approaches to the checkbox tag which should meet all your checkbox needs.

Approach One - When the bound value is of type java.lang.Boolean, the
input(checkbox) is marked as 'checked' if the bound value is true. The value
attribute corresponds to the resolved value of the setValue(Object) value property.

Approach Two - When the bound value is of type array or java.util.Collection, the
input(checkbox) is marked as 'checked' if the configured setValue(Object) value is
present in the bound Collection.

Approach Three - For any other bound value type, the input(checkbox) is marked as
'checked' if the configured setValue(Object) is equal to the bound value.

Note that regardless of the approach, the same HTML structure is generated. Below is an
HTML snippet of some checkboxes:

What you might not expect to see is the additional hidden field after each checkbox.
When a checkbox in an HTML page is not checked, its value will not be sent to the
server as part of the HTTP request parameters once the form is submitted, so we need a
workaround for this quirk in HTML in order for Spring form data binding to work. The
checkbox tag follows the existing Spring convention of including a hidden parameter
prefixed by an underscore ("_") for each checkbox. By doing this, you are effectively
telling Spring that "the checkbox was visible in the form and I want my object to
which the form data will be bound to reflect the state of the checkbox no matter what".

The checkboxes tag

This tag renders multiple HTML 'input' tags with type 'checkbox'.

Building on the example from the previous checkbox tag section. Sometimes you prefer
not to have to list all the possible hobbies in your JSP page. You would rather provide
a list at runtime of the available options and pass that in to the tag. That is the
purpose of the checkboxes tag. You pass in an Array, a List or a Map containing
the available options in the "items" property. Typically the bound property is a
collection so it can hold multiple values selected by the user. Below is an example of
the JSP using this tag:

<form:form><table><tr><td>Interests:</td><td><%-- Property is of an array or of type java.util.Collection --%><form:checkboxespath="preferences.interests"items="${interestList}"/></td></tr></table></form:form>

This example assumes that the "interestList" is a List available as a model attribute
containing strings of the values to be selected from. In the case where you use a Map,
the map entry key will be used as the value and the map entry’s value will be used as
the label to be displayed. You can also use a custom object where you can provide the
property names for the value using "itemValue" and the label using "itemLabel".

The radiobutton tag

This tag renders an HTML 'input' tag with type 'radio'.

A typical usage pattern will involve multiple tag instances bound to the same property
but with different values.

The radiobuttons tag

This tag renders multiple HTML 'input' tags with type 'radio'.

Just like the checkboxes tag above, you might want to pass in the available options as
a runtime variable. For this usage you would use the radiobuttons tag. You pass in an
Array, a List or a Map containing the available options in the "items" property.
In the case where you use a Map, the map entry key will be used as the value and the map
entry’s value will be used as the label to be displayed. You can also use a custom
object where you can provide the property names for the value using "itemValue" and the
label using "itemLabel".

As the example shows, the combined usage of an option tag with the options tag
generates the same standard HTML, but allows you to explicitly specify a value in the
JSP that is for display only (where it belongs) such as the default string in the
example: "-- Please Select".

The items attribute is typically populated with a collection or array of item objects.
itemValue and itemLabel simply refer to bean properties of those item objects, if
specified; otherwise, the item objects themselves will be stringified. Alternatively,
you may specify a Map of items, in which case the map keys are interpreted as option
values and the map values correspond to option labels. If itemValue and/or itemLabel
happen to be specified as well, the item value property will apply to the map key and
the item label property will apply to the map value.

<form:form><table><tr><td>First Name:</td><td><form:inputpath="firstName"/></td><%-- Show errors for firstName field --%><td><form:errorspath="firstName"/></td></tr><tr><td>Last Name:</td><td><form:inputpath="lastName"/></td><%-- Show errors for lastName field --%><td><form:errorspath="lastName"/></td></tr><tr><tdcolspan="3"><inputtype="submit"value="Save Changes"/></td></tr></table></form:form>

If we submit a form with empty values in the firstName and lastName fields, this is
what the HTML would look like:

<formmethod="POST"><spanname="*.errors"class="errorBox">Field is required.<br/>Field is required.</span><table><tr><td>First Name:</td><td><inputname="firstName"type="text"value=""/></td><td><spanname="firstName.errors">Field is required.</span></td></tr><tr><td>Last Name:</td><td><inputname="lastName"type="text"value=""/></td><td><spanname="lastName.errors">Field is required.</span></td></tr><tr><tdcolspan="3"><inputtype="submit"value="Save Changes"/></td></tr></table></form>

The spring-form.tld tag library descriptor (TLD) is included in the spring-webmvc.jar.
For a comprehensive reference on individual tags, browse the
API reference
or see the tag library description.

HTTP method conversion

A key principle of REST is the use of the Uniform Interface. This means that all
resources (URLs) can be manipulated using the same four HTTP methods: GET, PUT, POST,
and DELETE. For each method, the HTTP specification defines the exact semantics. For
instance, a GET should always be a safe operation, meaning that is has no side effects,
and a PUT or DELETE should be idempotent, meaning that you can repeat these operations
over and over again, but the end result should be the same. While HTTP defines these
four methods, HTML only supports two: GET and POST. Fortunately, there are two possible
workarounds: you can either use JavaScript to do your PUT or DELETE, or simply do a POST
with the 'real' method as an additional parameter (modeled as a hidden input field in an
HTML form). This latter trick is what Spring’s HiddenHttpMethodFilter does. This
filter is a plain Servlet Filter and therefore it can be used in combination with any
web framework (not just Spring MVC). Simply add this filter to your web.xml, and a POST
with a hidden _method parameter will be converted into the corresponding HTTP method
request.

To support HTTP method conversion the Spring MVC form tag was updated to support setting
the HTTP method. For example, the following snippet taken from the updated Petclinic
sample

HTML5 tags

Starting with Spring 3, the Spring form tag library allows entering dynamic attributes,
which means you can enter any HTML5 specific attributes.

In Spring 3.1, the form input tag supports entering a type attribute other than 'text'.
This is intended to allow rendering new HTML5 specific input types such as 'email',
'date', 'range', and others. Note that entering type='text' is not required since 'text'
is the default type.

1.10.6. Tiles

It is possible to integrate Tiles - just as any other view technology - in web
applications using Spring. The following describes in a broad way how to do this.

This section focuses on Spring’s support for Tiles v3 in the
org.springframework.web.servlet.view.tiles3 package.

Dependencies

To be able to use Tiles, you have to add a dependency on Tiles version 3.0.1 or higher
and its transitive dependencies
to your project.

Configuration

To be able to use Tiles, you have to configure it using files containing definitions
(for basic information on definitions and other Tiles concepts, please have a look at
http://tiles.apache.org). In Spring this is done using the TilesConfigurer. Have a
look at the following piece of example ApplicationContext configuration:

As you can see, there are five files containing definitions, which are all located in
the 'WEB-INF/defs' directory. At initialization of the WebApplicationContext, the
files will be loaded and the definitions factory will be initialized. After that has
been done, the Tiles includes in the definition files can be used as views within your
Spring web application. To be able to use the views you have to have a ViewResolver
just as with any other view technology used with Spring. Below you can find two
possibilities, the UrlBasedViewResolver and the ResourceBundleViewResolver.

You can specify locale specific Tiles definitions by adding an underscore and then
the locale. For example:

...
welcomeView.(class)=org.springframework.web.servlet.view.tiles3.TilesView
welcomeView.url=welcome (this is the name of a Tiles definition)
vetsView.(class)=org.springframework.web.servlet.view.tiles3.TilesView
vetsView.url=vetsView (again, this is the name of a Tiles definition)
findOwnersForm.(class)=org.springframework.web.servlet.view.JstlView
findOwnersForm.url=/WEB-INF/jsp/findOwners.jsp
...

As you can see, when using the ResourceBundleViewResolver, you can easily mix
different view technologies.

Note that the TilesView class supports JSTL (the JSP Standard Tag Library) out of the
box.

SimpleSpringPreparerFactory and SpringBeanPreparerFactory

As an advanced feature, Spring also supports two special Tiles PreparerFactory
implementations. Check out the Tiles documentation for details on how to use
ViewPreparer references in your Tiles definition files.

Specify SimpleSpringPreparerFactory to autowire ViewPreparer instances based on
specified preparer classes, applying Spring’s container callbacks as well as applying
configured Spring BeanPostProcessors. If Spring’s context-wide annotation-config has
been activated, annotations in ViewPreparer classes will be automatically detected and
applied. Note that this expects preparer classes in the Tiles definition files, just
like the default PreparerFactory does.

Specify SpringBeanPreparerFactory to operate on specified preparer names instead
of classes, obtaining the corresponding Spring bean from the DispatcherServlet’s
application context. The full bean creation process will be in the control of the Spring
application context in this case, allowing for the use of explicit dependency injection
configuration, scoped beans etc. Note that you need to define one Spring bean definition
per preparer name (as used in your Tiles definitions).

1.10.7. RSS, Atom

Both AbstractAtomFeedView and AbstractRssFeedView inherit from the base class
AbstractFeedView and are used to provide Atom and RSS Feed views respectfully. They
are based on java.net’s ROME project and are located in the
package org.springframework.web.servlet.view.feed.

AbstractAtomFeedView requires you to implement the buildFeedEntries() method and
optionally override the buildFeedMetadata() method (the default implementation is
empty), as shown below.

The buildFeedItems() and buildFeedEntires() methods pass in the HTTP request in case
you need to access the Locale. The HTTP response is passed in only for the setting of
cookies or other HTTP headers. The feed will automatically be written to the response
object after the method returns.

For an example of creating an Atom view please refer to Alef Arendsen’s Spring Team Blog
entry.

1.10.8. PDF, Excel

Introduction

Returning an HTML page isn’t always the best way for the user to view the model output,
and Spring makes it simple to generate a PDF document or an Excel spreadsheet
dynamically from the model data. The document is the view and will be streamed from the
server with the correct content type to (hopefully) enable the client PC to run their
spreadsheet or PDF viewer application in response.

In order to use Excel views, you need to add the Apache POI library to your classpath,
and for PDF generation preferably the OpenPDF library.

Use the latest versions of the underlying document generation libraries if possible.
In particular, we strongly recommend OpenPDF (e.g. OpenPDF 1.0.5) instead of the
outdated original iText 2.1.7 since it is actively maintained and fixes an important
vulnerability for untrusted PDF content.

Configuration

Document based views are handled in an almost identical fashion to XSLT views, and the
following sections build upon the previous one by demonstrating how the same controller
used in the XSLT example is invoked to render the same model as both a PDF document and
an Excel spreadsheet (which can also be viewed or manipulated in Open Office).

View definition

First, let’s amend the views.properties file (or xml equivalent) and add a simple view
definition for both document types. The entire file now looks like this with the XSLT
view shown from earlier:

If you want to start with a template spreadsheet or a fillable PDF form to add your
model data to, specify the location as the 'url' property in the view definition

Controller

The controller code we’ll use remains exactly the same from the XSLT example earlier
other than to change the name of the view to use. Of course, you could be clever and
have this selected based on a URL parameter or some other logic - proof that Spring
really is very good at decoupling the views from the controllers!

Excel views

Exactly as we did for the XSLT example, we’ll subclass suitable abstract classes in
order to implement custom behavior in generating our output documents. For Excel, this
involves writing a subclass of
org.springframework.web.servlet.view.document.AbstractExcelView (for Excel files
generated by POI) or org.springframework.web.servlet.view.document.AbstractJExcelView
(for JExcelApi-generated Excel files) and implementing the buildExcelDocument() method.

Here’s the complete listing for our POI Excel view which displays the word list from the
model map in consecutive rows of the first column of a new spreadsheet:

Note the differences between the APIs. We’ve found that the JExcelApi is somewhat more
intuitive, and furthermore, JExcelApi has slightly better image-handling capabilities.
There have been memory problems with large Excel files when using JExcelApi however.

If you now amend the controller such that it returns xl as the name of the view (
return new ModelAndView("xl", map);) and run your application again, you should find
that the Excel spreadsheet is created and downloaded automatically when you request the
same page as before.

PDF views

The PDF version of the word list is even simpler. This time, the class extends
org.springframework.web.servlet.view.document.AbstractPdfView and implements the
buildPdfDocument() method as follows:

Once again, amend the controller to return the pdf view with return new
ModelAndView("pdf", map);, and reload the URL in your application. This time a PDF
document should appear listing each of the words in the model map.

1.10.9. Jackson

JSON

The MappingJackson2JsonView uses the Jackson library’s ObjectMapper to render the response
content as JSON. By default, the entire contents of the model map (with the exception of
framework-specific classes) will be encoded as JSON. For cases where the contents of the
map need to be filtered, users may specify a specific set of model attributes to encode
via the RenderedAttributes property. The extractValueFromSingleKeyModel property may
also be used to have the value in single-key models extracted and serialized directly
rather than as a map of model attributes.

JSON mapping can be customized as needed through the use of Jackson’s provided
annotations. When further control is needed, a custom ObjectMapper can be injected
through the ObjectMapper property for cases where custom JSON
serializers/deserializers need to be provided for specific types.

JSONP is supported and automatically enabled when
the request has a query parameter named jsonp or callback. The JSONP query parameter
name(s) could be customized through the jsonpParameterNames property.

XML

The MappingJackson2XmlView uses the
Jackson XML extension's XmlMapper
to render the response content as XML. If the model contains multiples entries, the
object to be serialized should be set explicitly using the modelKey bean property.
If the model contains a single entry, it will be serialized automatically.

XML mapping can be customized as needed through the use of JAXB or Jackson’s provided
annotations. When further control is needed, a custom XmlMapper can be injected
through the ObjectMapper property for cases where custom XML
serializers/deserializers need to be provided for specific types.

1.10.10. XML

The MarshallingView uses an XML Marshaller defined in the org.springframework.oxm
package to render the response content as XML. The object to be marshalled can be set
explicitly using MarhsallingView's modelKey bean property. Alternatively, the view
will iterate over all model properties and marshal the first type that is supported
by the Marshaller. For more information on the functionality in the
org.springframework.oxm package refer to the chapter Marshalling XML using O/X
Mappers.

1.10.11. XSLT

XSLT is a transformation language for XML and is popular as a view technology within web
applications. XSLT can be a good choice as a view technology if your application
naturally deals with XML, or if your model can easily be converted to XML. The following
section shows how to produce an XML document as model data and have it transformed with
XSLT in a Spring Web MVC application.

This example is a trivial Spring application that creates a list of words in the
Controller and adds them to the model map. The map is returned along with the view
name of our XSLT view. See Annotated Controllers for details of Spring Web MVC’s
Controller interface. The XSLT Controller will turn the list of words into a simple XML
document ready for transformation.

Beans

Configuration is standard for a simple Spring application.
The MVC configuration has to define a XsltViewResolver bean and
regular MVC annotation configuration.

So far we’ve only created a DOM document and added it to the Model map. Note that you
can also load an XML file as a Resource and use it instead of a custom DOM document.

Of course, there are software packages available that will automatically 'domify'
an object graph, but within Spring, you have complete flexibility to create the DOM
from your model in any way you choose. This prevents the transformation of XML playing
too great a part in the structure of your model data which is a danger when using tools
to manage the domification process.

Next, XsltViewResolver will resolve the "home" XSLT template file and merge the
DOM document into it to generate our view.

Transformation

Finally, the XsltViewResolver will resolve the "home" XSLT template file and merge the
DOM document into it to generate our view. As shown in the XsltViewResolver
configuration, XSLT templates live in the war file in the 'WEB-INF/xsl' directory
and end with a "xslt" file extension.

1.11.2. MVC Config API

In XML check attributes and sub-elements of <mvc:annotation-driven/>. You can
view the Spring MVC XML schema or use
the code completion feature of your IDE to discover what attributes and
sub-elements are available.

1.11.3. Type conversion

By default formatters for Number and Date types are installed, including support for
the @NumberFormat and @DateTimeFormat annotations. Full support for the Joda-Time
formatting library is also installed if Joda-Time is present on the classpath.

1.11.4. Validation

By default if Bean Validation is present
on the classpath — e.g. Hibernate Validator, the LocalValidatorFactoryBean is registered
as a global Validator for use with @Valid and Validated on
controller method arguments.

1.11.6. Content Types

By default the URL path extension is checked first — with json, xml, rss, and atom
registered as known extensions depending on classpath dependencies, and the "Accept" header
is checked second.

Consider changing those defaults to Accept header only and if you must use URL-based
content type resolution consider the query parameter strategy over the path extensions. See
Suffix match and Suffix match and RFD for
more details.

1.11.7. Message Converters

Customization of HttpMessageConverter can be achieved in Java config by overriding
configureMessageConverters()
if you want to replace the default converters created by Spring MVC, or by overriding
extendMessageConverters()
if you just want to customize them or add additional converters to the default ones.

Below is an example that adds Jackson JSON and XML converters with a customized
ObjectMapper instead of default ones:

In this example,
Jackson2ObjectMapperBuilder
is used to create a common configuration for both MappingJackson2HttpMessageConverter and
MappingJackson2XmlHttpMessageConverter with indentation enabled, a customized date format
and the registration of
jackson-module-parameter-names
that adds support for accessing parameter names (feature added in Java 8).

This builder customizes Jackson’s default properties with the following ones:

1.11.8. View Controllers

This is a shortcut for defining a ParameterizableViewController that immediately
forwards to a view when invoked. Use it in static cases when there is no Java controller
logic to execute before the view generates the response.

An example of forwarding a request for "/" to a view called "home" in Java:

1.11.10. Static Resources

This option provides a convenient way to serve static resources from a list of
Resource-based locations.

In the example below, given a request that starts with "/resources", the relative path is
used to find and serve static resources relative to "/public" under the web application
root or on the classpath under "/static". The resources are served with a 1-year future
expiration to ensure maximum use of the browser cache and a reduction in HTTP requests
made by the browser. The Last-Modified header is also evaluated and if present a 304
status code is returned.

The VersionResourceResolver can be used for versioned resource URLs based on an MD5 hash
computed from the content, a fixed application version, or other. A
ContentVersionStrategy (MD5 hash) is a good choice with some notable exceptions such as
JavaScript resources used with a module loader.

You can use ResourceUrlProvider to rewrite URLs and apply the full chain of resolvers and
transformers — e.g. to insert versions. The MVC config provides a ResourceUrlProvider
bean so it can be injected into others. You can also make the rewrite transparent with the
ResourceUrlEncodingFilter for Thymeleaf, JSPs, FreeMarker, and others with URL tags that
rely on HttpServletResponse#encodeURL.

WebJars is also supported via WebJarsResourceResolver
and automatically registered when "org.webjars:webjars-locator" is present on the
classpath. The resolver can re-write URLs to include the version of the jar and can also
match to incoming URLs without versions — e.g. "/jquery/jquery.min.js" to
"/jquery/1.2.0/jquery.min.js".

1.11.11. Default Servlet

This allows for mapping the DispatcherServlet to "/" (thus overriding the mapping
of the container’s default Servlet), while still allowing static resource requests to be
handled by the container’s default Servlet. It configures a
DefaultServletHttpRequestHandler with a URL mapping of "/**" and the lowest priority
relative to other URL mappings.

This handler will forward all requests to the default Servlet. Therefore it is important
that it remains last in the order of all other URL HandlerMappings. That will be the
case if you use <mvc:annotation-driven> or alternatively if you are setting up your
own customized HandlerMapping instance be sure to set its order property to a value
lower than that of the DefaultServletHttpRequestHandler, which is Integer.MAX_VALUE.

The caveat to overriding the "/" Servlet mapping is that the RequestDispatcher for the
default Servlet must be retrieved by name rather than by path. The
DefaultServletHttpRequestHandler will attempt to auto-detect the default Servlet for
the container at startup time, using a list of known names for most of the major Servlet
containers (including Tomcat, Jetty, GlassFish, JBoss, Resin, WebLogic, and WebSphere).
If the default Servlet has been custom configured with a different name, or if a
different Servlet container is being used where the default Servlet name is unknown,
then the default Servlet’s name must be explicitly provided as in the following example:

You can keep existing methods in WebConfig but you can now also override bean declarations
from the base class and you can still have any number of other WebMvcConfigurer's on
the classpath.

1.11.14. Advanced XML Config

The MVC namespace does not have an advanced mode. If you need to customize a property on
a bean that you can’t change otherwise, you can use the BeanPostProcessor lifecycle
hook of the Spring ApplicationContext:

1.12. HTTP/2

Servlet 4 containers are required to support HTTP/2 and Spring Framework 5 is compatible
with Servlet API 4. From a programming model perspective there is nothing specific that
applications need to do. However there are considerations related to server configuration.
For more details please check out the
HTTP/2 wiki page.

The Servlet API does expose one construct related to HTTP/2. The
javax.servlet.http.PushBuilder can used to proactively push resources to clients and it
is supported as a method argument to @RequestMapping methods.

2. REST Clients

This section describes options for client-side access to REST endpoints.

2.1. RestTemplate

RestTemplate is the original Spring REST client that follows a similar approach to other
template classes in the Spring Framework (e.g. JdbcTemplate, JmsTemplate, etc.) by
providing a list of parameterizable methods to perform HTTP requests.

RestTemplate has a synchronous API and relies on blocking I/O. This is okay for
client scenarios with low concurrency. In a server environment or when orchestrating a
sequence of remote calls, prefer using the WebClient which provides a more efficient
execution model including seamless support for streaming.

2.2. WebClient

WebClient is a reactive client that provides an alternative to the RestTemplate. It
exposes a functional, fluent API and relies on non-blocking I/O which allows it to support
high concurrency more efficiently (i.e. using a small number of threads) than the
RestTemplate. WebClient is a natural fit for streaming scenarios.

3. Testing

This section summarizes the options available in spring-test for Spring MVC applications.

Servlet API Mocks

Mock implementations of Servlet API contracts for unit testing controllers, filters, and
other web components. See Servlet API mock objects
for more details.

TestContext Framework

Support for loading Spring configuration in JUnit and TestNG tests including efficient
caching of the loaded configuration across test methods and support for loading a
WebApplicationContext with a MockServletContext.
See TestContext Framework for more details.

Spring MVC Test

A framework, also known as MockMvc, for testing annotated controllers through the
DispatcherServlet, i.e. supporting annotations and complete with Spring MVC
infrastructure, but without an HTTP server. See
Spring MVC Test for more details.

Client-side REST

spring-test provides a MockRestServiceServer that can be used as a mock server for
testing client-side code that internally uses the RestTemplate.
See Client REST Tests for more details.

WebTestClient

WebTestClient was built for testing WebFlux applications but it can also be used for
end-to-end integration testing, to any server, over an HTTP connection. It is a
non-blocking, reactive client and well suited for testing asynchronous and streaming
scenarios.

4. WebSockets

This part of the reference documentation covers support for Servlet stack, WebSocket
messaging that includes raw WebSocket interactions, WebSocket emulation via SockJS, and
pub-sub messaging via STOMP as a sub-protocol over WebSocket.

4.1. Introduction

The WebSocket protocol RFC 6455 provides a standardized
way to establish a full-duplex, two-way communication channel between client and server
over a single TCP connection. It is a different TCP protocol from HTTP but is designed to
work over HTTP, using ports 80 and 443 and allowing re-use of existing firewall rules.

A WebSocket interaction begins with an HTTP request that uses the HTTP "Upgrade" header
to upgrade, or in this case to switch, to the WebSocket protocol:

After a successful handshake the TCP socket underlying the HTTP upgrade request remains
open for both client and server to continue to send and receive messages.

A complete introduction of how WebSockets work is beyond the scope of this document.
Please read RFC 6455, the WebSocket chapter of HTML5, or one of many introductions and
tutorials on the Web.

Note that if a WebSocket server is running behind a web server (e.g. nginx) you will
likely need to configure it to pass WebSocket upgrade requests on to the WebSocket
server. Likewise if the application runs in a cloud environment, check the
instructions of the cloud provider related to WebSocket support.

4.1.1. HTTP vs WebSocket

Even though WebSocket is designed to be HTTP compatible and starts with an HTTP request,
it is important to understand that the two protocols lead to very different
architectures and application programming models.

In HTTP and REST, an application is modeled as many URLs. To interact with the application
clients access those URLs, request-response style. Servers route requests to the
appropriate handler based on the HTTP URL, method, and headers.

By contrast in WebSockets there is usually just one URL for the initial connect and
subsequently all application messages flow on that same TCP connection. This points to
an entirely different asynchronous, event-driven, messaging architecture.

WebSocket is also a low-level transport protocol which unlike HTTP does not prescribe
any semantics to the content of messages. That means there is no way to route or process
a message unless client and server agree on message semantics.

WebSocket clients and servers can negotiate the use of a higher-level, messaging protocol
(e.g. STOMP), via the "Sec-WebSocket-Protocol" header on the HTTP handshake request,
or in the absence of that they need to come up with their own conventions.

4.1.2. When to use it?

WebSockets can make a web page dynamic and interactive. However in many cases
a combination of Ajax and HTTP streaming and/or long polling could provide a simple and
effective solution.

For example news, mail, and social feeds need to update dynamically but it may be
perfectly okay to do so every few minutes. Collaboration, games, and financial apps on
the other hand need to be much closer to real time.

Latency alone is not a deciding factor. If the volume of messages is relatively low (e.g.
monitoring network failures) HTTP streaming or polling may provide an effective solution.
It is the combination of low latency, high frequency and high volume that make the best
case for the use WebSocket.

Keep in mind also that over the Internet, restrictive proxies outside your control,
may preclude WebSocket interactions either because they are not configured to pass on the
Upgrade header or because they close long lived connections that appear idle? This
means that the use of WebSocket for internal applications within the firewall is a more
straight-forward decision than it is for public facing applications.

The above is for use in Spring MVC applications and should be included in the
configuration of a DispatcherServlet. However, Spring’s WebSocket
support does not depend on Spring MVC. It is relatively simple to integrate a WebSocketHandler
into other HTTP serving environments with the help of
WebSocketHttpRequestHandler.

4.2.2. WebSocket Handshake

The easiest way to customize the initial HTTP WebSocket handshake request is through
a HandshakeInterceptor, which exposes "before" and "after" the handshake methods.
Such an interceptor can be used to preclude the handshake or to make any attributes
available to the WebSocketSession. For example, there is a built-in interceptor
for passing HTTP session attributes to the WebSocket session:

A more advanced option is to extend the DefaultHandshakeHandler that performs
the steps of the WebSocket handshake, including validating the client origin,
negotiating a sub-protocol, and others. An application may also need to use this
option if it needs to configure a custom RequestUpgradeStrategy in order to
adapt to a WebSocket server engine and version that is not yet supported
(also see Deployment for more on this subject).
Both the Java-config and XML namespace make it possible to configure a custom
HandshakeHandler.

Spring provides a WebSocketHandlerDecorator base class that can be used to decorate
a WebSocketHandler with additional behavior. Logging and exception handling
implementations are provided and added by default when using the WebSocket Java-config
or XML namespace. The ExceptionWebSocketHandlerDecorator catches all uncaught
exceptions arising from any WebSocketHandler method and closes the WebSocket
session with status 1011 that indicates a server error.

4.2.3. Deployment

The Spring WebSocket API is easy to integrate into a Spring MVC application where
the DispatcherServlet serves both HTTP WebSocket handshake as well as other
HTTP requests. It is also easy to integrate into other HTTP processing scenarios
by invoking WebSocketHttpRequestHandler. This is convenient and easy to
understand. However, special considerations apply with regards to JSR-356 runtimes.

The Java WebSocket API (JSR-356) provides two deployment mechanisms. The first
involves a Servlet container classpath scan (Servlet 3 feature) at startup; and
the other is a registration API to use at Servlet container initialization.
Neither of these mechanism makes it possible to use a single "front controller"
for all HTTP processing — including WebSocket handshake and all other HTTP
requests — such as Spring MVC’s DispatcherServlet.

This is a significant limitation of JSR-356 that Spring’s WebSocket support addresses
server-specific RequestUpgradeStrategy's even when running in a JSR-356 runtime.
Such strategies currently exist for Tomcat, Jetty, GlassFish, WebLogic, WebSphere, and
Undertow (and WildFly).

A request to overcome the above limitation in the Java WebSocket API has been
created and can be followed at
WEBSOCKET_SPEC-211.
Tomcat, Undertow and WebSphere provide their own API alternatives that
makes it possible to this, and it’s also possible with Jetty. We are hopeful
that more servers will follow do the same.

A secondary consideration is that Servlet containers with JSR-356 support are expected
to perform a ServletContainerInitializer (SCI) scan that can slow down application
startup, in some cases dramatically. If a significant impact is observed after an
upgrade to a Servlet container version with JSR-356 support, it should
be possible to selectively enable or disable web fragments (and SCI scanning)
through the use of the <absolute-ordering /> element in web.xml:

4.2.5. Allowed origins

As of Spring Framework 4.1.5, the default behavior for WebSocket and SockJS is to accept
only same origin requests. It is also possible to allow all or a specified list of origins.
This check is mostly designed for browser clients. There is nothing preventing other types
of clients from modifying the Origin header value (see
RFC 6454: The Web Origin Concept for more details).

The 3 possible behaviors are:

Allow only same origin requests (default): in this mode, when SockJS is enabled, the
Iframe HTTP response header X-Frame-Options is set to SAMEORIGIN, and JSONP
transport is disabled since it does not allow to check the origin of a request.
As a consequence, IE6 and IE7 are not supported when this mode is enabled.

Allow a specified list of origins: each provided allowed origin must start with http://
or https://. In this mode, when SockJS is enabled, both IFrame and JSONP based
transports are disabled. As a consequence, IE6 through IE9 are not supported when this
mode is enabled.

Allow all origins: to enable this mode, you should provide * as the allowed origin
value. In this mode, all transports are available.

WebSocket and SockJS allowed origins can be configured as shown bellow:

4.3. SockJS Fallback

Over the public Internet, restrictive proxies outside your control may preclude WebSocket
interactions either because they are not configured to pass on the Upgrade header or
because they close long lived connections that appear idle.

The solution to this problem is WebSocket emulation, i.e. attempting to use WebSocket
first and then falling back on HTTP-based techniques that emulate a WebSocket
interaction and expose the same application-level API.

On the Servlet stack the Spring Framework provides both server (and also client) support
for the SockJS protocol.

4.3.1. Overview

The goal of SockJS is to let applications use a WebSocket API but fall back to
non-WebSocket alternatives when necessary at runtime, i.e. without the need to
change application code.

SockJS server implementations including one in the Spring Framework spring-websocket module.

As of 4.1 spring-websocket also provides a SockJS Java client.

SockJS is designed for use in browsers. It goes to great lengths
to support a wide range of browser versions using a variety of techniques.
For the full list of SockJS transport types and browsers see the
SockJS client page. Transports
fall in 3 general categories: WebSocket, HTTP Streaming, and HTTP Long Polling.
For an overview of these categories see
this blog post.

The SockJS client begins by sending "GET /info" to
obtain basic information from the server. After that it must decide what transport
to use. If possible WebSocket is used. If not, in most browsers
there is at least one HTTP streaming option and if not then HTTP (long)
polling is used.

The WebSocket transport needs only a single HTTP request to do the WebSocket handshake.
All messages thereafter are exchanged on that socket.

HTTP transports require more requests. Ajax/XHR streaming for example relies on
one long-running request for server-to-client messages and additional HTTP POST
requests for client-to-server messages. Long polling is similar except it
ends the current request after each server-to-client send.

SockJS adds minimal message framing. For example the server sends the letter o
("open" frame) initially, messages are sent as a["message1","message2"]
(JSON-encoded array), the letter h ("heartbeat" frame) if no messages flow
for 25 seconds by default, and the letter c ("close" frame) to close the session.

To learn more, run an example in a browser and watch the HTTP requests.
The SockJS client allows fixing the list of transports so it is possible to
see each transport one at a time. The SockJS client also provides a debug flag
which enables helpful messages in the browser console. On the server side enable
TRACE logging for org.springframework.web.socket.
For even more detail refer to the SockJS protocol
narrated test.

The above is for use in Spring MVC applications and should be included in the
configuration of a DispatcherServlet. However, Spring’s WebSocket
and SockJS support does not depend on Spring MVC. It is relatively simple to
integrate into other HTTP serving environments with the help of
SockJsHttpRequestHandler.

On the browser side, applications can use the
sockjs-client (version 1.0.x) that
emulates the W3C WebSocket API and communicates with the server to select the best
transport option depending on the browser it’s running in. Review the
sockjs-client page and the list of
transport types supported by browser. The client also provides several
configuration options, for example, to specify which transports to include.

4.3.3. IE 8, 9

Internet Explorer 8 and 9 are and will remain common for some time. They are
a key reason for having SockJS. This section covers important
considerations about running in those browsers.

The SockJS client supports Ajax/XHR streaming in IE 8 and 9 via Microsoft’s
XDomainRequest.
That works across domains but does not support sending cookies.
Cookies are very often essential for Java applications.
However since the SockJS client can be used with many server
types (not just Java ones), it needs to know whether cookies matter.
If so the SockJS client prefers Ajax/XHR for streaming or otherwise it
relies on a iframe-based technique.

The very first "/info" request from the SockJS client is a request for
information that can influence the client’s choice of transports.
One of those details is whether the server application relies on cookies,
e.g. for authentication purposes or clustering with sticky sessions.
Spring’s SockJS support includes a property called sessionCookieNeeded.
It is enabled by default since most Java applications rely on the JSESSIONID
cookie. If your application does not need it, you can turn off this option
and the SockJS client should choose xdr-streaming in IE 8 and 9.

If you do use an iframe-based transport, and in any case, it is good to know
that browsers can be instructed to block the use of IFrames on a given page by
setting the HTTP response header X-Frame-Options to DENY,
SAMEORIGIN, or ALLOW-FROM <origin>. This is used to prevent
clickjacking.

Spring Security 3.2+ provides support for setting X-Frame-Options on every
response. By default the Spring Security Java config sets it to DENY.
In 3.2 the Spring Security XML namespace does not set that header by default
but may be configured to do so, and in the future it may set it by default.

If your application adds the X-Frame-Options response header (as it should!)
and relies on an iframe-based transport, you will need to set the header value to
SAMEORIGIN or ALLOW-FROM <origin>. Along with that the Spring SockJS
support also needs to know the location of the SockJS client because it is loaded
from the iframe. By default the iframe is set to download the SockJS client
from a CDN location. It is a good idea to configure this option to
a URL from the same origin as the application.

In Java config this can be done as shown below. The XML namespace provides a
similar option via the <websocket:sockjs> element:

During initial development, do enable the SockJS client devel mode that prevents
the browser from caching SockJS requests (like the iframe) that would otherwise
be cached. For details on how to enable it see the
SockJS client page.

4.3.4. Heartbeats

The SockJS protocol requires servers to send heartbeat messages to preclude proxies
from concluding a connection is hung. The Spring SockJS configuration has a property
called heartbeatTime that can be used to customize the frequency. By default a
heartbeat is sent after 25 seconds assuming no other messages were sent on that
connection. This 25 seconds value is in line with the following
IETF recommendation for public Internet applications.

When using STOMP over WebSocket/SockJS, if the STOMP client and server negotiate
heartbeats to be exchanged, the SockJS heartbeats are disabled.

The Spring SockJS support also allows configuring the TaskScheduler to use
for scheduling heartbeats tasks. The task scheduler is backed by a thread pool
with default settings based on the number of available processors. Applications
should consider customizing the settings according to their specific needs.

4.3.5. Client disconnects

HTTP streaming and HTTP long polling SockJS transports require a connection to remain
open longer than usual. For an overview of these techniques see
this blog post.

In Servlet containers this is done through Servlet 3 async support that
allows exiting the Servlet container thread processing a request and continuing
to write to the response from another thread.

A specific issue is that the Servlet API does not provide notifications for a client
that has gone away, see SERVLET_SPEC-44.
However, Servlet containers raise an exception on subsequent attempts to write
to the response. Since Spring’s SockJS Service supports sever-sent heartbeats (every
25 seconds by default), that means a client disconnect is usually detected within that
time period or earlier if messages are sent more frequently.

As a result network IO failures may occur simply because a client has disconnected, which
can fill the log with unnecessary stack traces. Spring makes a best effort to identify
such network failures that represent client disconnects (specific to each server) and log
a minimal message using the dedicated log category DISCONNECTED_CLIENT_LOG_CATEGORY
defined in AbstractSockJsSession. If you need to see the stack traces, set that
log category to TRACE.

4.3.6. SockJS and CORS

If you allow cross-origin requests (see Allowed origins), the SockJS protocol
uses CORS for cross-domain support in the XHR streaming and polling transports. Therefore
CORS headers are added automatically unless the presence of CORS headers in the response
is detected. So if an application is already configured to provide CORS support, e.g.
through a Servlet Filter, Spring’s SockJsService will skip this part.

It is also possible to disable the addition of these CORS headers via the
suppressCors property in Spring’s SockJsService.

The following is the list of headers and values expected by SockJS:

"Access-Control-Allow-Origin" - initialized from the value of the "Origin" request header.

"Access-Control-Allow-Credentials" - always set to true.

"Access-Control-Request-Headers" - initialized from values from the equivalent request header.

4.3.7. SockJsClient

A SockJS Java client is provided in order to connect to remote SockJS endpoints without
using a browser. This can be especially useful when there is a need for bidirectional
communication between 2 servers over a public network, i.e. where network proxies may
preclude the use of the WebSocket protocol. A SockJS Java client is also very useful
for testing purposes, for example to simulate a large number of concurrent users.

The SockJS Java client supports the "websocket", "xhr-streaming", and "xhr-polling"
transports. The remaining ones only make sense for use in a browser.

The WebSocketTransport can be configured with:

StandardWebSocketClient in a JSR-356 runtime

JettyWebSocketClient using the Jetty 9+ native WebSocket API

Any implementation of Spring’s WebSocketClient

An XhrTransport by definition supports both "xhr-streaming" and "xhr-polling" since
from a client perspective there is no difference other than in the URL used to connect
to the server. At present there are two implementations:

SockJS uses JSON formatted arrays for messages. By default Jackson 2 is used and needs
to be on the classpath. Alternatively you can configure a custom implementation of
SockJsMessageCodec and configure it on the SockJsClient.

To use the SockJsClient for simulating a large number of concurrent users you will
need to configure the underlying HTTP client (for XHR transports) to allow a sufficient
number of connections and threads. For example with Jetty:

4.4. STOMP

The WebSocket protocol defines two types of messages, text and binary, but their
content is undefined. The defines a mechanism for client and server to negotiate a
sub-protocol — i.e. a higher level messaging protocol, to use on top of WebSocket to
define what kind of messages each can send, what is the format and content for each
message, and so on. The use of a sub-protocol is optional but either way client and
server will need to agree on some protocol that defines message content.

4.4.1. Overview

STOMP is a simple,
text-oriented messaging protocol that was originally created for scripting languages
such as Ruby, Python, and Perl to connect to enterprise message brokers. It is
designed to address a minimal subset of commonly used messaging patterns. STOMP can be
used over any reliable, 2-way streaming network protocol such as TCP and WebSocket.
Although STOMP is a text-oriented protocol, message payloads can be
either text or binary.

STOMP is a frame based protocol whose frames are modeled on HTTP. The structure
of a STOMP frame:

COMMAND
header1:value1
header2:value2
Body^@

Clients can use the SEND or SUBSCRIBE commands to send or subscribe for
messages along with a "destination" header that describes what the
message is about and who should receive it. This enables a simple
publish-subscribe mechanism that can be used to send messages through the broker
to other connected clients or to send messages to the server to request that
some work be performed.

When using Spring’s STOMP support, the Spring WebSocket application acts
as the STOMP broker to clients. Messages are routed to @Controller message-handling
methods or to a simple, in-memory broker that keeps track of subscriptions and
broadcasts messages to subscribed users. You can also configure Spring to work
with a dedicated STOMP broker (e.g. RabbitMQ, ActiveMQ, etc) for the actual
broadcasting of messages. In that case Spring maintains
TCP connections to the broker, relays messages to it, and also passes messages
from it down to connected WebSocket clients. Thus Spring web applications can
rely on unified HTTP-based security, common validation, and a familiar programming
model message-handling work.

Here is an example of a client subscribing to receive stock quotes which
the server may emit periodically e.g. via a scheduled task sending messages
through a SimpMessagingTemplate to the broker:

SUBSCRIBE
id:sub-1
destination:/topic/price.stock.*
^@

Here is an example of a client sending a trade request, which the server
may handle through an @MessageMapping method and later on, after the execution,
broadcast a trade confirmation message and details down to the client:

The meaning of a destination is intentionally left opaque in the STOMP spec. It can
be any string, and it’s entirely up to STOMP servers to define the semantics and
the syntax of the destinations that they support. It is very common, however, for
destinations to be path-like strings where "/topic/.." implies publish-subscribe
(one-to-many) and "/queue/" implies point-to-point (one-to-one) message
exchanges.

STOMP servers can use the MESSAGE command to broadcast messages to all subscribers.
Here is an example of a server sending a stock quote to a subscribed client:

It is important to know that a server cannot send unsolicited messages. All messages
from a server must be in response to a specific client subscription, and the
"subscription-id" header of the server message must match the "id" header of the
client subscription.

The above overview is intended to provide the most basic understanding of the
STOMP protocol. It is recommended to review the protocol
specification in full.

4.4.2. Benefits

Use of STOMP as a sub-protocol enables the Spring Framework and Spring Security to
provide a richer programming model vs using raw WebSockets. The same point can be
made about how HTTP vs raw TCP and how it enables Spring MVC and other web frameworks
to provide rich functionality. The following is a list of benefits:

No need to invent a custom messaging protocol and message format.

STOMP clients are available including a Java client
in the Spring Framework.

Message brokers such as RabbitMQ, ActiveMQ, and others can be used (optionally) to
manage subscriptions and broadcast messages.

Application logic can be organized in any number of @Controller's and messages
routed to them based on the STOMP destination header vs handling raw WebSocket messages
with a single WebSocketHandler for a given connection.

Use Spring Security to secure messages based on STOMP destinations and message types.

4.4.3. Enable STOMP

STOMP over WebSocket support is available in the spring-messaging and the
spring-websocket modules. Once you have those dependencies, you can expose a STOMP
endpoints, over WebSocket with SockJS Fallback, as shown below:

For the built-in, simple broker the "/topic" and "/queue" prefixes do not have any special
meaning. They’re merely a convention to differentiate between pub-sub vs point-to-point
messaging (i.e. many subscribers vs one consumer). When using an external broker, please
check the STOMP page of the broker to understand what kind of STOMP destinations and
prefixes it supports.

To connect from a browser, for SockJS you can use the
sockjs-client. For STOMP many applications have
used the jmesnil/stomp-websocket library
(also known as stomp.js) which is feature complete and has been used in production for
years but is no longer maintained. At present the
JSteunou/webstomp-client is the most
actively maintained and evolving successor of that library and the example code below
is based on it:

Note that the stompClient above does not need to specify login and passcode headers.
Even if it did, they would be ignored, or rather overridden, on the server side. See the
sections Connect to Broker and
Authentication for more information on authentication.

4.4.4. Flow of Messages

Once a STOMP endpoint is exposed, the Spring application becomes a STOMP broker for
connected clients. This section describes the flow of messages on the server side.

The spring-messaging module contains foundational support for messaging applications
that originated in Spring Integration and was
later extracted and incorporated into the Spring Framework for broader use across many
Spring projects and application scenarios.
Below is a list of a few of the available messaging abstractions:

Message — simple representation for a message including headers and payload.

Both the Java config (i.e. @EnableWebSocketMessageBroker) and the XML namespace config
(i.e. <websocket:message-broker>) use the above components to assemble a message
workflow. The diagram below shows the components used when the simple, built-in message
broker is enabled:

There are 3 message channels in the above diagram:

"clientInboundChannel" — for passing messages received from WebSocket clients.

"brokerChannel" — for sending messages to the message broker from within
server-side, application code.

The next diagram shows the components used when an external broker (e.g. RabbitMQ)
is configured for managing subscriptions and broadcasting messages:

The main difference in the above diagram is the use of the "broker relay" for passing
messages up to the external STOMP broker over TCP, and for passing messages down from the
broker to subscribed clients.

When messages are received from a WebSocket connectin, they’re decoded to STOMP frames,
then turned into a Spring Message representation, and sent to the
"clientInboundChannel" for further processing. For example STOMP messages whose
destination header starts with "/app" may be routed to @MessageMapping methods in
annotated controllers, while "/topic" and "/queue" messages may be routed directly
to the message broker.

An annotated @Controller handling a STOMP message from a client may send a message to
the message broker through the "brokerChannel", and the broker will broadcast the
message to matching subscribers through the "clientOutboundChannel". The same
controller can also do the same in response to HTTP requests, so a client may perform an
HTTP POST and then an @PostMapping method can send a message to the message broker
to broadcast to subscribed clients.

Let’s trace the flow through a simple example. Given the following server setup:

Client connects to "http://localhost:8080/portfolio" and once a WebSocket connection
is established, STOMP frames begin to flow on it.

Client sends SUBSCRIBE frame with destination header "/topic/greeting". Once received
and decoded, the message is sent to the "clientInboundChannel", then routed to the
message broker which stores the client subscription.

Client sends SEND frame to "/app/greeting". The "/app" prefix helps to route it to
annotated controllers. After the "/app" prefix is stripped, the remaining "/greeting"
part of the destination is mapped to the @MessageMapping method in GreetingController.

The value returned from GreetingController is turned into a Spring Message with
a payload based on the return value and a default destination header of
"/topic/greeting" (derived from the input destination with "/app" replaced by
"/topic"). The resulting message is sent to the "brokerChannel" and handled
by the message broker.

The message broker finds all matching subscribers, and sends a MESSAGE frame to each
through the "clientOutboundChannel" from where messages are encoded as STOMP frames
and sent on the WebSocket connection.

The next section provides more details on annotated methods including the
kinds of arguments and return values supported.

4.4.5. Annotated Controllers

Applications can use annotated @Controller classes to handle messages from clients.
Such classes can declare @MessageMapping, @SubscribeMapping, and @ExceptionHandler
methods as described next.

@MessageMapping

The @MessageMapping annotation can be used on methods to route messages based on their
destination. It is supported at the method level as well as at the type level. At type
level @MessageMapping is used to express shared mappings across all methods in a
controller.

By default destination mappings are expected to be Ant-style, path patterns, e.g. "/foo*",
"/foo/**". The patterns include support for template variables, e.g. "/foo/{id}", that can
be referenced with @DestinationVariable method arguments.

Applications can choose to switch to a dot-separated destination convention.
See Dot as Separator.

@MessageMapping methods can have flexible signatures with the following arguments:

Method argument

Description

Message

For access to the complete message.

MessageHeaders

For access to the headers within the Message.

MessageHeaderAccessor, SimpMessageHeaderAccessor, StompHeaderAccessor

For access to the headers via typed accessor methods.

@Payload

For access to the payload of the message, converted (e.g. from JSON) via a configured
MessageConverter.

The presence of this annotation is not required since it is assumed by default if no
other argument is matched.

Payload arguments may be annotated with @javax.validation.Valid or Spring’s @Validated
in order to be automatically validated.

@Header

For access to a specific header value along with type conversion using an
org.springframework.core.convert.converter.Converter if necessary.

@Headers

For access to all headers in the message. This argument must be assignable to
java.util.Map.

@DestinationVariable

For access to template variables extracted from the message destination.
Values will be converted to the declared method argument type as necessary.

java.security.Principal

Reflects the user logged in at the time of the WebSocket HTTP handshake.

When an @MessageMapping method returns a value, by default the value is serialized to
a payload through a configured MessageConverter, and then sent as a Message to the
"brokerChannel" from where it is broadcast to subscribers. The destination of the
outbound message is the same as that of the inbound message but prefixed with "/topic".

You can use the @SendTo method annotation to customize the destination to send
the payload to. @SendTo can also be used at the class level to share a default target
destination to send messages to. @SendToUser is an variant for sending messages only to
the user associated with a message. See User Destinations for details.

The return value from an @MessageMapping method may be wrapped with ListenableFuture,
CompletableFuture, or CompletionStage in order to produce the payload asynchronously.

As an alternative to returning a payload from an @MessageMapping method you can also
send messages using the SimpMessagingTemplate, which is also how return values are
handled under the covers. See Send Messages.

@SubscribeMapping

The @SubscribeMapping annotation is used in combination with @MessageMapping in order
to narrow the mapping to subscription messages. In such scenarios, the @MessageMapping
annotation specifies the destination while @SubscribeMapping indicates interest in
subscription messages only.

An @SubscribeMapping method is generally no different from any @MessageMapping
method with respect to mapping and input arguments. For example you can combine it with a
type-level @MessageMapping to express a shared destination prefix, and you can use the
same method arguments as any @MessageMapping` method.

The key difference with @SubscribeMapping is that the return value of the method is
serialized as a payload and sent, not to the "brokerChannel" but to the
"clientOutboundChannel", effectively replying directly to the client rather than
broadcasting through the broker. This is useful for implementing one-off, request-reply
message exchanges, and never holding on to the subscription. A common scenario for this
pattern is application initialization when data must be loaded and presented.

A @SubscribeMapping method can also be annotated with @SendTo in which case the
return value is sent to the "brokerChannel" with the explicitly specified target
destination.

@MessageExceptionHandler

An application can use @MessageExceptionHandler methods to handle exceptions from
@MessageMapping methods. Exceptions of interest can be declared in the annotation
itself, or through a method argument if you want to get access to the exception instance:

@MessageExceptionHandler methods support flexible method signatures and support the same
method argument types and return values as @MessageMapping methods.

Typically @MessageExceptionHandler methods apply within the @Controller class (or
class hierarchy) they are declared in. If you want such methods to apply more globally,
across controllers, you can declare them in a class marked with @ControllerAdvice.
This is comparable to similar support in Spring MVC.

4.4.6. Send Messages

What if you want to send messages to connected clients from any part of the
application? Any application component can send messages to the "brokerChannel".
The easiest way to do that is to have a SimpMessagingTemplate injected, and
use it to send messages. Typically it should be easy to have it injected by
type, for example:

Applications can also use dot-separated destinations (vs slash).
See Dot as Separator.

4.4.8. External Broker

The simple broker is great for getting started but supports only a subset of
STOMP commands (e.g. no acks, receipts, etc.), relies on a simple message
sending loop, and is not suitable for clustering. As an alternative, applications
can upgrade to using a full-featured message broker.

Check the STOMP documentation for your message broker of choice (e.g.
RabbitMQ,
ActiveMQ, etc.), install the broker,
and run it with STOMP support enabled. Then enable the STOMP broker relay in the
Spring configuration instead of the simple broker.

The "STOMP broker relay" in the above configuration is a Spring
MessageHandler
that handles messages by forwarding them to an external message broker.
To do so it establishes TCP connections to the broker, forwards all
messages to it, and then forwards all messages received
from the broker to clients through their WebSocket sessions. Essentially
it acts as a "relay" that forwards messages in both directions.

Furthermore, application components (e.g. HTTP request handling methods,
business services, etc.) can also send messages to the broker relay, as described
in Send Messages, in order to broadcast messages to
subscribed WebSocket clients.

4.4.9. Connect to Broker

A STOMP broker relay maintains a single "system" TCP connection to the broker.
This connection is used for messages originating from the server-side application
only, not for receiving messages. You can configure the STOMP credentials
for this connection, i.e. the STOMP frame login and passcode headers. This
is exposed in both the XML namespace and the Java config as the
systemLogin/systemPasscode properties with default values guest/guest.

The STOMP broker relay also creates a separate TCP connection for every connected
WebSocket client. You can configure the STOMP credentials to use for all TCP
connections created on behalf of clients. This is exposed in both the XML namespace
and the Java config as the clientLogin/clientPasscode properties with default
values guest/guest.

The STOMP broker relay always sets the login and passcode headers on every CONNECT
frame that it forwards to the broker on behalf of clients. Therefore WebSocket clients
need not set those headers; they will be ignored. As the Authentication
section explains, instead WebSocket clients should rely on HTTP authentication to protect
the WebSocket endpoint and establish the client identity.

The STOMP broker relay also sends and receives heartbeats to and from the message
broker over the "system" TCP connection. You can configure the intervals for sending
and receiving heartbeats (10 seconds each by default). If connectivity to the broker
is lost, the broker relay will continue to try to reconnect, every 5 seconds,
until it succeeds.

Any Spring bean can implement ApplicationListener<BrokerAvailabilityEvent> in order
to receive notifications when the "system" connection to the broker is lost and
re-established. For example a Stock Quote service broadcasting stock quotes can
stop trying to send messages when there is no active "system" connection.

By default, the STOMP broker relay always connects, and reconnects as needed if
connectivity is lost, to the same host and port. If you wish to supply multiple addresses,
on each attempt to connect, you can configure a supplier of addresses, instead of a
fixed host and port. For example:

The STOMP broker relay can also be configured with a virtualHost property.
The value of this property will be set as the host header of every CONNECT frame
and may be useful for example in a cloud environment where the actual host to which
the TCP connection is established is different from the host providing the
cloud-based STOMP service.

4.4.10. Dot as Separator

When messages are routed to @MessageMapping methods, they’re matched with
AntPathMatcher and by default patterns are expected to use slash "/" as separator.
This is a good convention in a web applications and similar to HTTP URLs. However if
you are more used to messaging conventions, you can switch to using dot "." as separator.

In the example above we did not change the prefixes on the "broker relay" because those
depend entirely on the external message broker. Check the STOMP documentation pages of
the broker you’re using to see what conventions it supports for the destination header.

The "simple broker" on the other hand does rely on the configured PathMatcher so if
you switch the separator that will also apply to the broker and the way matches
destinations from a message to patterns in subscriptions.

4.4.11. Authentication

Every STOMP over WebSocket messaging session begins with an HTTP request — that can be a request to upgrade to WebSockets (i.e. a WebSocket handshake)
or in the case of SockJS fallbacks a series of SockJS HTTP transport requests.

Web applications already have authentication and authorization in place to
secure HTTP requests. Typically a user is authenticated via Spring Security
using some mechanism such as a login page, HTTP basic authentication, or other.
The security context for the authenticated user is saved in the HTTP session
and is associated with subsequent requests in the same cookie-based session.

Therefore for a WebSocket handshake, or for SockJS HTTP transport requests,
typically there will already be an authenticated user accessible via
HttpServletRequest#getUserPrincipal(). Spring automatically associates that user
with a WebSocket or SockJS session created for them and subsequently with all
STOMP messages transported over that session through a user header.

In short there is nothing special a typical web application needs to do above
and beyond what it already does for security. The user is authenticated at
the HTTP request level with a security context maintained through a cookie-based
HTTP session which is then associated with WebSocket or SockJS sessions created
for that user and results in a user header stamped on every Message flowing
through the application.

Note that the STOMP protocol does have a "login" and "passcode" headers
on the CONNECT frame. Those were originally designed for and are still needed
for example for STOMP over TCP. However for STOMP over WebSocket by default
Spring ignores authorization headers at the STOMP protocol level and assumes
the user is already authenticated at the HTTP transport level and expects that
the WebSocket or SockJS session contain the authenticated user.

Spring Security provides
WebSocket sub-protocol authorization
that uses a ChannelInterceptor to authorize messages based on the user header in them.
Also Spring Session provides a
WebSocket integration
that ensures the user HTTP session does not expire when the WebSocket session is still active.

4.4.12. Token Authentication

Spring Security OAuth
provides support for token based security including JSON Web Token (JWT).
This can be used as the authentication mechanism in Web applications
including STOMP over WebSocket interactions just as described in the previous
section, i.e. maintaining identity through a cookie-based session.

At the same time cookie-based sessions are not always the best fit for example
in applications that don’t wish to maintain a server-side session at all or in
mobile applications where it’s common to use headers for authentication.

The WebSocket protocol RFC 6455
"doesn’t prescribe any particular way that servers can authenticate clients during
the WebSocket handshake." In practice however browser clients can only use standard
authentication headers (i.e. basic HTTP authentication) or cookies and cannot for example
provide custom headers. Likewise the SockJS JavaScript client does not provide
a way to send HTTP headers with SockJS transport requests, see
sockjs-client issue 196.
Instead it does allow sending query parameters that can be used to send a token
but that has its own drawbacks, for example as the token may be inadvertently
logged with the URL in server logs.

The above limitations are for browser-based clients and do not apply to the
Spring Java-based STOMP client which does support sending headers with both
WebSocket and SockJS requests.

Therefore applications that wish to avoid the use of cookies may not have any good
alternatives for authentication at the HTTP protocol level. Instead of using cookies
they may prefer to authenticate with headers at the STOMP messaging protocol level
There are 2 simple steps to doing that:

Use the STOMP client to pass authentication header(s) at connect time.

Process the authentication header(s) with a ChannelInterceptor.

Below is the example server-side configuration to register a custom authentication
interceptor. Note that an interceptor only needs to authenticate and set
the user header on the CONNECT Message. Spring will note and save the authenticated
user and associate it with subsequent STOMP messages on the same session:

Also note that when using Spring Security’s authorization for messages, at present
you will need to ensure that the authentication ChannelInterceptor config is ordered
ahead of Spring Security’s. This is best done by declaring the custom interceptor in
its own implementation of WebSocketMessageBrokerConfigurer marked with
@Order(Ordered.HIGHEST_PRECEDENCE + 99).

4.4.13. User Destinations

An application can send messages targeting a specific user, and Spring’s STOMP support
recognizes destinations prefixed with "/user/" for this purpose.
For example, a client might subscribe to the destination "/user/queue/position-updates".
This destination will be handled by the UserDestinationMessageHandler and
transformed into a destination unique to the user session,
e.g. "/queue/position-updates-user123". This provides the convenience of subscribing
to a generically named destination while at the same time ensuring no collisions
with other users subscribing to the same destination so that each user can receive
unique stock position updates.

On the sending side messages can be sent to a destination such as
"/user/{username}/queue/position-updates", which in turn will be translated
by the UserDestinationMessageHandler into one or more destinations, one for each
session associated with the user. This allows any component within the application to
send messages targeting a specific user without necessarily knowing anything more
than their name and the generic destination. This is also supported through an
annotation as well as a messaging template.

For example, a message-handling method can send messages to the user associated with
the message being handled through the @SendToUser annotation (also supported on
the class-level to share a common destination):

If the user has more than one session, by default all of the sessions subscribed
to the given destination are targeted. However sometimes, it may be necessary to
target only the session that sent the message being handled. This can be done by
setting the broadcast attribute to false, for example:

While user destinations generally imply an authenticated user, it isn’t required
strictly. A WebSocket session that is not associated with an authenticated user
can subscribe to a user destination. In such cases the @SendToUser annotation
will behave exactly the same as with broadcast=false, i.e. targeting only the
session that sent the message being handled.

It is also possible to send a message to user destinations from any application
component by injecting the SimpMessagingTemplate created by the Java config or
XML namespace, for example (the bean name is "brokerMessagingTemplate" if required
for qualification with @Qualifier):

When using user destinations with an external message broker, check the broker
documentation on how to manage inactive queues, so that when the user session is
over, all unique user queues are removed. For example, RabbitMQ creates auto-delete
queues when destinations like /exchange/amq.direct/position-updates are used.
So in that case the client could subscribe to /user/exchange/amq.direct/position-updates.
Similarly, ActiveMQ has
configuration options
for purging inactive destinations.

In a multi-application server scenario a user destination may remain unresolved because
the user is connected to a different server. In such cases you can configure a
destination to broadcast unresolved messages to so that other servers have a chance to try.
This can be done through the userDestinationBroadcast property of the
MessageBrokerRegistry in Java config and the user-destination-broadcast attribute
of the message-broker element in XML.

4.4.14. Events and Interception

Several ApplicationContext events (listed below) are published and can be
received by implementing Spring’s ApplicationListener interface.

BrokerAvailabilityEvent — indicates when the broker becomes available/unavailable.
While the "simple" broker becomes available immediately on startup and remains so while
the application is running, the STOMP "broker relay" may lose its connection
to the full featured broker, for example if the broker is restarted. The broker relay
has reconnect logic and will re-establish the "system" connection to the broker
when it comes back, hence this event is published whenever the state changes from connected
to disconnected and vice versa. Components using the SimpMessagingTemplate should
subscribe to this event and avoid sending messages at times when the broker is not
available. In any case they should be prepared to handle MessageDeliveryException
when sending a message.

SessionConnectEvent — published when a new STOMP CONNECT is received
indicating the start of a new client session. The event contains the message representing the
connect including the session id, user information (if any), and any custom headers the client
may have sent. This is useful for tracking client sessions. Components subscribed
to this event can wrap the contained message using SimpMessageHeaderAccessor or
StompMessageHeaderAccessor.

SessionConnectedEvent — published shortly after a SessionConnectEvent when the
broker has sent a STOMP CONNECTED frame in response to the CONNECT. At this point the
STOMP session can be considered fully established.

SessionSubscribeEvent — published when a new STOMP SUBSCRIBE is received.

SessionUnsubscribeEvent — published when a new STOMP UNSUBSCRIBE is received.

SessionDisconnectEvent — published when a STOMP session ends. The DISCONNECT may
have been sent from the client, or it may also be automatically generated when the
WebSocket session is closed. In some cases this event may be published more than once
per session. Components should be idempotent with regard to multiple disconnect events.

When using a full-featured broker, the STOMP "broker relay" automatically reconnects the
"system" connection in case the broker becomes temporarily unavailable. Client connections
however are not automatically reconnected. Assuming heartbeats are enabled, the client
will typically notice the broker is not responding within 10 seconds. Clients need to
implement their own reconnect logic.

Furthermore, an application can directly intercept every incoming and outgoing message by
registering a ChannelInterceptor on the respective message channel. For example
to intercept inbound messages:

In the above example StandardWebSocketClient could be replaced with SockJsClient
since that is also an implementation of WebSocketClient. The SockJsClient can
use WebSocket or HTTP-based transport as a fallback. For more details see
SockJsClient.

Next establish a connection and provide a handler for the STOMP session:

Once the session is established any payload can be sent and that will be
serialized with the configured MessageConverter:

session.send("/topic/foo", "payload");

You can also subscribe to destinations. The subscribe methods require a handler
for messages on the subscription and return a Subscription handle that can be
used to unsubscribe. For each received message the handler can specify the target
Object type the payload should be deserialized to:

To enable STOMP heartbeat configure WebSocketStompClient with a TaskScheduler
and optionally customize the heartbeat intervals, 10 seconds for write inactivity
which causes a heartbeat to be sent and 10 seconds for read inactivity which
closes the connection.

When using WebSocketStompClient for performance tests to simulate thousands
of clients from the same machine consider turning off heartbeats since each
connection schedules its own heartbeat tasks and that’s not optimized for a
a large number of clients running on the same machine.

The STOMP protocol also supports receipts where the client must add a "receipt"
header to which the server responds with a RECEIPT frame after the send or
subscribe are processed. To support this the StompSession offers
setAutoReceipt(boolean) that causes a "receipt" header to be
added on every subsequent send or subscribe.
Alternatively you can also manually add a "receipt" header to the StompHeaders.
Both send and subscribe return an instance of Receiptable
that can be used to register for receipt success and failure callbacks.
For this feature the client must be configured with a TaskScheduler
and the amount of time before a receipt expires (15 seconds by default).

Note that StompSessionHandler itself is a StompFrameHandler which allows
it to handle ERROR frames in addition to the handleException callback for
exceptions from the handling of messages, and handleTransportError for
transport-level errors including ConnectionLostException.

4.4.16. WebSocket Scope

Each WebSocket session has a map of attributes. The map is attached as a header to
inbound client messages and may be accessed from a controller method, for example:

It is also possible to declare a Spring-managed bean in the websocket scope.
WebSocket-scoped beans can be injected into controllers and any channel interceptors
registered on the "clientInboundChannel". Those are typically singletons and live
longer than any individual WebSocket session. Therefore you will need to use a
scope proxy mode for WebSocket-scoped beans:

As with any custom scope, Spring initializes a new MyBean instance the first
time it is accessed from the controller and stores the instance in the WebSocket
session attributes. The same instance is returned subsequently until the session
ends. WebSocket-scoped beans will have all Spring lifecycle methods invoked as
shown in the examples above.

4.4.17. Performance

There is no silver bullet when it comes to performance. Many factors may
affect it including the size of messages, the volume, whether application
methods perform work that requires blocking, as well as external factors
such as network speed and others. The goal of this section is to provide
an overview of the available configuration options along with some thoughts
on how to reason about scaling.

In a messaging application messages are passed through channels for asynchronous
executions backed by thread pools. Configuring such an application requires
good knowledge of the channels and the flow of messages. Therefore it is
recommended to review Flow of Messages.

The obvious place to start is to configure the thread pools backing the
"clientInboundChannel" and the "clientOutboundChannel". By default both
are configured at twice the number of available processors.

If the handling of messages in annotated methods is mainly CPU bound then the
number of threads for the "clientInboundChannel" should remain close to the
number of processors. If the work they do is more IO bound and requires blocking
or waiting on a database or other external system then the thread pool size
will need to be increased.

ThreadPoolExecutor has 3 important properties. Those are the core and
the max thread pool size as well as the capacity for the queue to store
tasks for which there are no available threads.

A common point of confusion is that configuring the core pool size (e.g. 10)
and max pool size (e.g. 20) results in a thread pool with 10 to 20 threads.
In fact if the capacity is left at its default value of Integer.MAX_VALUE
then the thread pool will never increase beyond the core pool size since
all additional tasks will be queued.

Please review the Javadoc of ThreadPoolExecutor to learn how these
properties work and understand the various queuing strategies.

On the "clientOutboundChannel" side it is all about sending messages to WebSocket
clients. If clients are on a fast network then the number of threads should
remain close to the number of available processors. If they are slow or on
low bandwidth they will take longer to consume messages and put a burden on the
thread pool. Therefore increasing the thread pool size will be necessary.

While the workload for the "clientInboundChannel" is possible to predict — after all it is based on what the application does — how to configure the
"clientOutboundChannel" is harder as it is based on factors beyond
the control of the application. For this reason there are two additional
properties related to the sending of messages. Those are the "sendTimeLimit"
and the "sendBufferSizeLimit". Those are used to configure how long a
send is allowed to take and how much data can be buffered when sending
messages to a client.

The general idea is that at any given time only a single thread may be used
to send to a client. All additional messages meanwhile get buffered and you
can use these properties to decide how long sending a message is allowed to
take and how much data can be buffered in the mean time. Please review the
Javadoc and documentation of the XML schema for this configuration for
important additional details.

The WebSocket transport configuration shown above can also be used to configure the
maximum allowed size for incoming STOMP messages. Although in theory a WebSocket
message can be almost unlimited in size, in practice WebSocket servers impose
limits — for example, 8K on Tomcat and 64K on Jetty. For this reason STOMP clients
such as the JavaScript webstomp-client
and others split larger STOMP messages at 16K boundaries and send them as multiple
WebSocket messages thus requiring the server to buffer and re-assemble.

Spring’s STOMP over WebSocket support does this so applications can configure the
maximum size for STOMP messages irrespective of WebSocket server specific message
sizes. Do keep in mind that the WebSocket message size will be automatically
adjusted if necessary to ensure they can carry 16K WebSocket messages at a
minimum.

An important point about scaling is using multiple application instances.
Currently it is not possible to do that with the simple broker.
However when using a full-featured broker such as RabbitMQ, each application
instance connects to the broker and messages broadcast from one application
instance can be broadcast through the broker to WebSocket clients connected
through any other application instances.

4.4.18. Monitoring

When using @EnableWebSocketMessageBroker or <websocket:message-broker> key
infrastructure components automatically gather stats and counters that provide
important insight into the internal state of the application. The configuration
also declares a bean of type WebSocketMessageBrokerStats that gathers all
available information in one place and by default logs it at INFO level once
every 30 minutes. This bean can be exported to JMX through Spring’s
MBeanExporter for viewing at runtime, for example through JDK’s jconsole.
Below is a summary of the available information.

Client WebSocket Sessions

Current

indicates how many client sessions there are
currently with the count further broken down by WebSocket vs HTTP
streaming and polling SockJS sessions.

Total

indicates how many total sessions have been established.

Abnormally Closed

Connect Failures

these are sessions that got established but were
closed after not having received any messages within 60 seconds. This is
usually an indication of proxy or network issues.

Send Limit Exceeded

sessions closed after exceeding the configured send
timeout or the send buffer limits which can occur with slow clients
(see previous section).

Transport Errors

sessions closed after a transport error such as
failure to read or write to a WebSocket connection or
HTTP request/response.

STOMP Frames

the total number of CONNECT, CONNECTED, and DISCONNECT frames
processed indicating how many clients connected on the STOMP level. Note that
the DISCONNECT count may be lower when sessions get closed abnormally or when
clients close without sending a DISCONNECT frame.

STOMP Broker Relay

TCP Connections

indicates how many TCP connections on behalf of client
WebSocket sessions are established to the broker. This should be equal to the
number of client WebSocket sessions + 1 additional shared "system" connection
for sending messages from within the application.

STOMP Frames

the total number of CONNECT, CONNECTED, and DISCONNECT frames
forwarded to or received from the broker on behalf of clients. Note that a
DISCONNECT frame is sent to the broker regardless of how the client WebSocket
session was closed. Therefore a lower DISCONNECT frame count is an indication
that the broker is pro-actively closing connections, may be because of a
heartbeat that didn’t arrive in time, an invalid input frame, or other.

Client Inbound Channel

stats from thread pool backing the "clientInboundChannel"
providing insight into the health of incoming message processing. Tasks queueing
up here is an indication the application may be too slow to handle messages.
If there I/O bound tasks (e.g. slow database query, HTTP request to 3rd party
REST API, etc) consider increasing the thread pool size.

Client Outbound Channel

stats from the thread pool backing the "clientOutboundChannel"
providing insight into the health of broadcasting messages to clients. Tasks
queueing up here is an indication clients are too slow to consume messages.
One way to address this is to increase the thread pool size to accommodate the
number of concurrent slow clients expected. Another option is to reduce the
send timeout and send buffer size limits (see the previous section).

SockJS Task Scheduler

stats from thread pool of the SockJS task scheduler which
is used to send heartbeats. Note that when heartbeats are negotiated on the
STOMP level the SockJS heartbeats are disabled.

4.4.19. Testing

There are two main approaches to testing applications using Spring’s STOMP over
WebSocket support. The first is to write server-side tests verifying the functionality
of controllers and their annotated message handling methods. The second is to write
full end-to-end tests that involve running a client and a server.

The two approaches are not mutually exclusive. On the contrary each has a place
in an overall test strategy. Server-side tests are more focused and easier to write
and maintain. End-to-end integration tests on the other hand are more complete and
test much more, but they’re also more involved to write and maintain.

The simplest form of server-side tests is to write controller unit tests. However
this is not useful enough since much of what a controller does depends on its
annotations. Pure unit tests simply can’t test that.

Ideally controllers under test should be invoked as they are at runtime, much like
the approach to testing controllers handling HTTP requests using the Spring MVC Test
framework. i.e. without running a Servlet container but relying on the Spring Framework
to invoke the annotated controllers. Just like with Spring MVC Test here there are two
two possible alternatives, either using a "context-based" or "standalone" setup:

Load the actual Spring configuration with the help of the
Spring TestContext framework, inject "clientInboundChannel" as a test field, and
use it to send messages to be handled by controller methods.

Manually set up the minimum Spring framework infrastructure required to invoke
controllers (namely the SimpAnnotationMethodMessageHandler) and pass messages for
controllers directly to it.

The second approach is to create end-to-end integration tests. For that you will need
to run a WebSocket server in embedded mode and connect to it as a WebSocket client
sending WebSocket messages containing STOMP frames.
The tests for the stock portfolio
sample application also demonstrates this approach using Tomcat as the embedded
WebSocket server and a simple STOMP client for test purposes.

5. Other Web Frameworks

5.1. Introduction

This chapter details Spring’s integration with third party web frameworks.

One of the core value propositions of the Spring Framework is that of enabling
choice. In a general sense, Spring does not force one to use or buy into any
particular architecture, technology, or methodology (although it certainly recommends
some over others). This freedom to pick and choose the architecture, technology, or
methodology that is most relevant to a developer and their development team is
arguably most evident in the web area, where Spring provides its own web framework
(Spring MVC), while at the same time providing integration with a number of
popular third party web frameworks.

5.2. Common config

Before diving into the integration specifics of each supported web framework, let us
first take a look at the Spring configuration that is not specific to any one web
framework. (This section is equally applicable to Spring’s own web framework, Spring
MVC.)

One of the concepts (for want of a better word) espoused by (Spring’s) lightweight
application model is that of a layered architecture. Remember that in a 'classic'
layered architecture, the web layer is but one of many layers; it serves as one of the
entry points into a server side application and it delegates to service objects
(facades) defined in a service layer to satisfy business specific (and
presentation-technology agnostic) use cases. In Spring, these service objects, any other
business-specific objects, data access objects, etc. exist in a distinct 'business
context', which contains no web or presentation layer objects (presentation objects
such as Spring MVC controllers are typically configured in a distinct 'presentation
context'). This section details how one configures a Spring container (a
WebApplicationContext) that contains all of the 'business beans' in one’s application.

On to specifics: all that one need do is to declare a
ContextLoaderListener
in the standard Java EE servlet web.xml file of one’s web application, and add a
contextConfigLocation<context-param/> section (in the same file) that defines which
set of Spring XML configuration files to load.

If you don’t specify the contextConfigLocation context parameter, the
ContextLoaderListener will look for a file called /WEB-INF/applicationContext.xml to
load. Once the context files are loaded, Spring creates a
WebApplicationContext
object based on the bean definitions and stores it in the ServletContext of the web
application.

All Java web frameworks are built on top of the Servlet API, and so one can use the
following code snippet to get access to this 'business context' ApplicationContext
created by the ContextLoaderListener.

The
WebApplicationContextUtils
class is for convenience, so you don’t have to remember the name of the ServletContext
attribute. Its getWebApplicationContext() method will return null if an object
doesn’t exist under the WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE
key. Rather than risk getting NullPointerExceptions in your application, it’s better
to use the getRequiredWebApplicationContext() method. This method throws an exception
when the ApplicationContext is missing.

Once you have a reference to the WebApplicationContext, you can retrieve beans by
their name or type. Most developers retrieve beans by name and then cast them to one of
their implemented interfaces.

Fortunately, most of the frameworks in this section have simpler ways of looking up
beans. Not only do they make it easy to get beans from a Spring container, but they also
allow you to use dependency injection on their controllers. Each web framework section
has more detail on its specific integration strategies.

5.3. JSF

JavaServer Faces (JSF) is the JCP’s standard component-based, event-driven web user
interface framework. As of Java EE 5, it is an official part of the Java EE umbrella.

For a popular JSF runtime as well as for popular JSF component libraries, check out the
Apache MyFaces project. The MyFaces project also provides
common JSF extensions such as MyFaces Orchestra:
a Spring-based JSF extension that provides rich conversation scope support.

Spring Web Flow 2.0 provides rich JSF support through its newly established Spring Faces
module, both for JSF-centric usage (as described in this section) and for Spring-centric
usage (using JSF views within a Spring MVC dispatcher). Check out the
Spring Web Flow website for details!

The key element in Spring’s JSF integration is the JSF ELResolver mechanism.

5.3.1. Spring Bean Resolver

SpringBeanFacesELResolver is a JSF 1.2+ compliant ELResolver implementation,
integrating with the standard Unified EL as used by JSF 1.2 and JSP 2.1. Like
SpringBeanVariableResolver, it delegates to the Spring’s 'business context'
WebApplicationContextfirst, then to the default resolver of the underlying JSF
implementation.

5.3.2. FacesContextUtils

A custom VariableResolver works well when mapping one’s properties to beans
in faces-config.xml, but at times one may need to grab a bean explicitly. The
FacesContextUtils
class makes this easy. It is similar to WebApplicationContextUtils, except that it
takes a FacesContext parameter rather than a ServletContext parameter.

5.4. Apache Struts 2.x

Invented by Craig McClanahan, Struts is an open source project
hosted by the Apache Software Foundation. At the time, it greatly simplified the
JSP/Servlet programming paradigm and won over many developers who were using proprietary
frameworks. It simplified the programming model, it was open source (and thus free as in
beer), and it had a large community, which allowed the project to grow and become popular
among Java web developers.

Check out the Struts
Spring Plugin for the
built-in Spring integration shipped with Struts.

5.5. Tapestry 5.x

While Spring has its own powerful web layer, there are a number of unique
advantages to building an enterprise Java application using a combination of Tapestry
for the web user interface and the Spring container for the lower layers.